Difference Algebra (Algebra and Applications)

(0)

(iii) There exists a p-tuple (r1 , . . . , rp ) ∈ Zp such that Mr1 ,...,rp = 0 if ri < (0) ri for at least one index i (1 ≤ i ≤ p). (iv) Dr1 ,...,rp Ms1 ,...,sp ⊆ Mr1 +s1 ,...,rp +sp for any p-tuples (r1 , . . . , rp ), (s1 , . . . , sp ) ∈ Zp . If every vector K-space Mr1 ,...,rp is finite-dimensional and there exists an element (h1 , . . . , hp ) ∈ Zp such that Dr1 ,...,rp Mh1 ,...,hp = Mr1 +h1 ,...,rp +hp for any (r1 , . . . , rp ) ∈ Np , the p-dimensional filtration {Mr1 ,...,rp |(r1 , . . . , rp ) ∈ Zp } is called excellent.

CHAPTER 3. DIFFERENCE MODULES

168

It is easy to see that if  z1 , . . . , zk is a finite system of generators of a k vector σ-K-space M , then { i=1 Dr1 ,...,rp zi |(r1 , . . . , rp ) ∈ Zp } is an excellent p-dimensional filtration of M . Let K be a difference field with a basic set σ = {α1 , . . . , αn }, T the free commutative semigroup generated by σ, and D the ring of σ-operators over K. Furthermore, we assume that a partition (3.3.1) of the set σ is fixed. A vector σ-K-space which is a free module over the ring of difference operators D is called a free σ-K-module or a free vector σ-K-space. If a free vector σ-K-space E is generated (as a free D-module) by a finite system of elements {e1 , . . . , em }, we say that E is a finitely generated free vector σ-K-space and call e1 , . . . , em free generators of E. In this case the elements of the form τ eν (τ ∈ T, 1 ≤ ν ≤ m) are called terms while the elements of the semigroup T are called monomilas. The set of all terms is denoted by T e. It is easy to see that this set generates E as a vector space over the field K. By the order of a term τ eν with respect to σi (1 ≤ i ≤ p) we mean the order of the monomial τ with respect to σi . A term u = τ  eµ is said to be a multiple of a term u = τ eν if µ = ν and τ  is a multiple of τ in the semigroup T . In this case we write u|u . (Clearly, u|u if and only if there exists τ  ∈ T such that
u = τ  u. Then we write τ  = uu .) The least common multiple of two terms u = τ1 ei and v = τ2 ej is defined as follows: $ 0, if i = j, lcm(u, v) = lcm(τ1 , τ2 )ei if i = j. We shall consider p orderings of the set T e that correspond to the orderings of the semigroup T introduced at the beginning of this section. These orderings are denoted by the same symbols <1 , . . . ,

(3.3.2)

where τ1 ei1 , . . . , τl eil are distinct elements of Te , and a1 , . . . , al are nonzero elements of K. Definition 3.3.2 Let f be a nonzero element of the D-module E written in the form (3.3.2.) and let τν eiν (1 ≤ ν ≤ p) be the greatest term of the set {τ1 ei1 , . . . , τl eil } with respect to an order
(j
)

possible that uf = uf (k)

integer ordk uf

for some distinct numbers j and j  .) The non-negative

is called the kth order of f and denoted by ordk f (k = 1, . . . , p). (k)

The coefficient of uf in f is said to be the k-leading coefficient of f ; it is denoted by lck (f ).

3.3. MULTIVARIABLE DIMENSION POLYNOMIALS

169

Definition 3.3.3 Let f, g ∈ E, g = 0, and let k, i1 , . . . , il be distinct elements of the set {1, . . . , p}. Then f is said to be (
ρ : E → Γe be defined by ρ(f ) = z1 2

(1)

d

p(f ) . . . zp−1 uf .

Definition 3.3.4 With the above notation, let N be a D-submodule of E. A fiobner basis nite set of nonzero elements G = {g1 , . . . , gr } ⊆ N is called a Gr¨ of N with respect to the orders <1 , . . . ,

and ordiν

(iν ) w (k) ug ug



w (k) ug

−1 (lck (g))

w (k)


g

ug

(i )

≤ ordiν uf ν (1 ≤ ν ≤ l).

Definition 3.3.6 Let f, h ∈ E and let G = {g1 , . . . , gr } be a finite set of nonzero elements of E. We say that f (
and write f −−−−−−−−−→ h if and only if there exists a sequence of elements g (1) , g (2) , . . . g


∈ G and a sequence of elements h1 , . . . , hq−1 ∈ E such that

g (1)

g (2)

g (q−1)

g (q)





f −−−−−−−−−→ h1 −−−−−−−−−→ . . . −−−−−−−−−→ hq−1 −−−−−−−−−→ h .

CHAPTER 3. DIFFERENCE MODULES

170

Theorem 3.3.7 With the above notation, let G = {g1 , . . . , gr } ⊆ E be a Gr¨ obner basis with respect to the orders <1 , . . . ,

g is (<1 , . . . ,

Furthermore, f  cannot contain any term of the form τ  uk (τ  ∈ T , 1 ≤ k ≤ r), that is greater than wf (with respect to <1 ) and satisfies the condition (j) (j) ordj (τ  uk ) ≤ ordj uf
for j = 2, . . . , p. Indeed, if the last inequality holds, then (j)

(j)

(1)

ordj (τ  uk ) ≤ ordj uf , so that the term τ  uk (1) u τ gi

(1) τ u gi

cannot appear in f . This term (1)

(1)

= = wf <j τ  uk . Thus, τ  ui cannot appear in τ gi either, since   cannot appear in f , whence the G-leader of f is strictly less (with respect to the order <1 ) than the G-leader of f . Applying the same procedure to the element f  and continuing in the same way, we obtain an element g ∈ E such that f − g is a linear combination of elements g1 , . . . , gr with coefficients in D  and g is (<1 , . . . ,

3.3. MULTIVARIABLE DIMENSION POLYNOMIALS ordj (

(j) w (1) ugi ) ugi

(j)

≤ ordj ug

171

for j = 2, . . . , p do

z:= the greatest (with respect to <1 ) of the terms w that satisfy the above conditions. (1) k:= the smallest number i for which ugi is the greatest (with respect to <1 ) 1-leader of an element gi ∈ G such that (1) (j) z ugi ) ≤ ordj ug for j = 2, . . . , p. ugi |z and ordj ( (1) ugi

−1 z z Qk := Qk + c(z) (1) (lc1 (gk ) (1) gk ugk ugk

−1 z z g := g − c(z) (1) (lc1 (gk ) (1) gk ugk

ugk

End The following example illustrates Algorithm 3.3.8. Example 3.3.9 Let K = Q(x, y) be the field of rational fractions in two variables x and y over Q treated as a difference field with a basic set σ = {α1 , α2 } where α1 f (x, y) = f (x + 1, y) and α2 f (x,
y) = f (x, y + 1) for every f (x, y) ∈ Q(x, y). We consider a partition σ = σ1 σ2 of the set σ where σ1 = {α1 } and σ2 = {α2 }. Let D be the ring of σ-operators over K, let E be the free left D-module with free generators e1 and e2 , and let G = {g1 , g2 } where g1 g2

=

x2 α1 α2 e1 + xα22 e2 − xyα1 e1 + y 2 e2 ,

=

2xα12 α2 e2 + α1 α22 e1 + yα2 e1 .

We are going to to apply Algorithm 3.3.8 to reduce the element f = xα12 α22 e1 + yα1 α32 e2 − xyα1 α2 e1 with respect to G. (1) (2) (1) (2) Notice that ug1 = α1 α2 e1 , ug1 = α22 e2 , ug2 = α12 α2 e2 , and ug2 = α1 α22 e1 . Furthermore, g1 and g2 are (<1 , <2 )-reduced with respect to each other. Indeed, (1) ug2 divides no term of g1 , so g1 is (<1 , <2 )-reduced with respect to g2 . The (1) (1) only term of g2 which is a multiple of ug1 is w = α1 α22 e1 . Since w = α2 ug1 (2) (2) and ord2 (α2 ug1 ) = ord2 (α23 ) = 3 > ord2 ug2 = 2, we obtain that g2 is (<1 , <2 )reduced with respect to g1 . According to Algorithm 3.3.8 the first step of the (<1 , <2 )-reduction of the element f with respect to G is as follows. 1 α1 α2 g1 (x + 1)2 % x (x + 1)2 α12 α22 e1 + (x + 1)α1 α23 e2 =f− 2 (x + 1) & − (x + 1)(y + 1)α12 α2 e1 + (y + 1)2 α1 α2 e2 

x(y + 1) 2 x(y + 1)2 x α1 α23 e2 + α1 α2 e1 − α1 α2 e2 . = y− x+1 x+1 (x + 1)2

f → f1 = f − x

CHAPTER 3. DIFFERENCE MODULES

172 (2)

(2)

(Notice that ord2 (α1 α2 ug1 ) ≤ ord2 uf ; both orders are equal to 3.) (1)

(1)

(2)

Since uf1 = α12 α2 e1 = α1 ug1 and the order of α1 ug1 = α1 α22 e2 with respect (2)

to σ2 does not exceed ord2 (uf1 ) = 3, the second step of the (<1 , <2 )-reduction is x(y + 1) 1 α1 g1 x + 1 (x + 1)2 

xy(y + 1) 2 x(y + 1) x(y + 1) α1 α23 e2 − = α e + y − α1 α22 e2 1 (x + 1)2 1 x+1 (x + 1)2

f1 → f2 = f1 −

−

x(y + 1)2 xy 2 (y + 1) α1 α2 e2 − α1 e2 . 2 (x + 1) (x + 1)3 (1)

(1)

Since no term of f2 is divisible by ug1 = α1 α2 e1 or by ug2 = α12 α2 e2 , the element f2 is (<1 , <2 )-reduced with respect to G. The proof of Theorem 3.3.7 shows that if G is a Gr¨ obner basis of a D-submodule N of E, then a reduction step described in Definition 3.3.5 (with some g ∈ G) can be applied to every nonzero element f ∈ N . As a result of such a step, we obtain an element of N whose G-leader is strictly less (with respect to <1 ) than the G-leader of f . This observation leads to the following statement. obner basis of a D-submodule Proposition 3.3.10 Let G = {g1 , . . . , gr } be a Gr¨ N of E with respect to the orders <1 , . . . ,

(i) f ∈ N if and only if f −−−−−−−−→ 0 . <1 ,<2 ,...,
(ii) If f ∈ N and f is (<1 , <2 , . . . ,

(k)

(k)

 −

−1

(k)

lcm(uf , ug ) uf

(k)

(lck (f ))

(k)

ug

(k)

(k)

−1

(k)

lcm(uf , ug )

(k)

lcm(uf , ug )

(lck (g))

f

uf

(k)

(k)

lcm(uf , ug ) (k)

g

ug

is called the kth S-polynomial of f and g. With the above notation, one can obtain the following two statements that generalize the corresponding properties of the classical Gr¨ obner basis. Proposition 3.3.12 Let f, g1 , . . . , gr be nonzero elements in E (r ≥ 1) and let r  ci ωi gi where ωi ∈ T , ci ∈ K (1 ≤ i ≤ r). Let k ∈ {1, . . . , p} and for any f= i=1

3.3. MULTIVARIABLE DIMENSION POLYNOMIALS (k)

(k)

(k)

173 (k)

ν, j ∈ {1, . . . , r}, let uνj = lcm(ugν , ugj ). Furthermore, suppose that ω1 ug1 = (k)

(k)

· · · = ωr ugr = u, ug
t s  

cνj θνj Sk (gν , gj )

ν=1 j=1

where θνj =

u (k)

and

uνj

(k)

(l)

(l)

θνj uSk (gν ,gj ) ≤l uf

θνj uSk (gν ,gj )
(1 ≤ ν ≤ s, 1 ≤ j ≤ t, l ∈ I).

PROOF. For every i = 1, . . . , r, let di = lck (ωi gi ) = ωi (lck (gi )). Since r (k) (k) (k) ω1 ug1 = · · · = ωr ugr = u and uf
r 

ci ωi gi =

i=1

r 

ci di hi = c1 d1 (h1 − h2 ) + (c1 d1 + c2 d2 )(h2 − h3 )

i=1

+ · · · + (c1 d1 + · · · + cr−1 dr−1 )(hr−1 − hr ). (The last sum should end with the term (c1 d1 +· · ·+cr dr )hr in order to represent an identity, but this term is equal to zero.) (k) (k) uνj uνj u For every ν, j ∈ {1, . . . , r}, ν = j, let τνj = (k) , γνj = (k) , and θνj = (k) u gν u gj uνj (k)

(k)

(k)

(since ugν | u and ugj | u, the term uνj divides u). Then % & θνj Sk (gν , gj ) = θνj (τνj (lck (gν )))−1 τνj gν − (γνj (lck (gj )))−1 γνj gj −1 u −1 u g − [θνj (γνj (lck (gj )))] g = [θνj (τνj (lck (gν )))] (k) ν (k) j u gν u gj −1

= [ων (lck (gν ))] = hν − hj .

−1

ων gν − [ωj (lck (gj ))]

ωj gj

It follows that f = c1 d1 θ12 Sk (g1 , g2 ) + (c1 d1 + c2 d2 )θ23 Sk (g2 , g3 ) + · · · + r−1  (k) (k) (k) ci di )θr−1,r Sk (gr−1 , gr ), θi,i+1 uSk (gi ,gi+1 ) = uθi,i+1 Sk (gi ,gi+1 ) = uhi −hi+1
(k)

(k)

u (since uhi = uhi+1 and lck (hi ) = lck (hi+1 ) = 1), and we have the inequalities (l)

(l)

θi,i+1 uSk (gi ,gi+1 ) ≤l uf for all i = 1, . . . , r − 1, l ∈ I. This completes the proof.  obner Theorem 3.3.13 With the above notation, let G = {g1 , . . . , gr } be a Gr¨ basis of a D-submodule N of E with respect to each of the following sequences of

CHAPTER 3. DIFFERENCE MODULES

174

orders:

Then G is a Gr¨ obner basis of N with respect to
f=

hi gi

(3.3.3)

i=1

where h1 , . . . , hr ∈ D, (k)

(k)

max
(j)

(3.3.4)

(j)

ordj (uhi u(j) gi ) ≤ ordj uf (j = k + 1, . . . , p).

(3.3.5)

(The symbol max
r 

Hi gi

(3.3.6)

i=1

where H1 , . . . , Hr ∈ D, (k+1) (k+1) u gi

max
(k+1)

| 1 ≤ i ≤ r} = uf

(3.3.7)

and (j)

(j)

ordj (uHi u(j) gi ) ≤ ordj uf (j = k + 2, . . . , p; i = 1, . . . , r).

(3.3.8)

Let us choose among all representations of f in the form (3.3.6) with conditions (3.3.7) and (3.3.8) a representation with the smallest (with respect to
r  i=1

Hi gi =

 (k) (k) uH ugi =u i

Hi gi +

 (k) (k) uH ugi
Hi gi

3.3. MULTIVARIABLE DIMENSION POLYNOMIALS or f=

 (k)



(k)

di uHi gi +

(k)

(k)

uH ugi =u

(k)

uH ugi =u

i



(k)

(Hi − di uHi )gi +

(k)

175

Hi gi

(3.3.9)

(k)

uH ugi
i

i

(k)

Notice that if u = uf , then the expansion (3.3.9) satisfies conditions (3.3.3) (k+1)

(3.3.5). Indeed, in this case uf

(k+1) (k+1) u gi

= max
| 1 ≤ i ≤ r} (see

(k+1) (k+1) (k+1) and (3.3.7)), hence max{ordk+1 (uHi ugi ) | 1 ≤ i ≤ r} ≤ ordk+1 uf (j) (j) (j) ordj (uHi ugi ) ≤ ordj uf for j = k + 2, . . . , p; i = 1, . . . , r ((see (3.3.8)). (k) (k) (k) Suppose that uf
of the first sum in (3.3.9) does not exceed u with respect to
(k)

uH ugi =u i

and for every j = k + 1, . . . , p, (j)

(k)

(j)

(j)

(j) ordj uf˜ ≤ maxi∈I {ordj (uHi u(j) gi )} ≤ maxi∈I {ordj (uHi ugi )} ≤ ordj uf

where I denotes the set of all indices i ∈ {1, . . . , r} that appear in (3.3.10). (k) (k) (k) u ∈ T. Let uνj = lcm(ugν , ugj ) for any ν, j ∈ I, ν = j and let τνj = (k) (k) (k)

uνj

(k)

(Since u = uHi ugi for every i ∈ I, uνj |u.) By Proposition 3.3.12, there exist elements cνj ∈ K such that  cνj τνj Sk (gν , gj ) f˜ = ν,j

where

(k)

(k)

uτνj Sk (gν ,gj )
(j)

(j)

ordj uτνj Sk (gν ,gj ) ≤ ordj uf˜ (j = k + 1, . . . , p). G

Since Sk (gν , gj ) −−−−−−−−−−→ 0, there exist qiνj ∈ D such that
Sk (gν , gj ) =

r 

qiνj gi

i=1

and (k)

(k) u(k) qiνj ugi ≤k uSk (gν ,gj ) , (l)

(l) ordl (u(l) qiνj ugi ) ≤l ordl uSk (gν ,gj )

(3.3.11)

CHAPTER 3. DIFFERENCE MODULES

176

for l = k + 1, . . . , p. Thus, for any indices ν, j in the sum (3.3.11) we have τνj Sk (gν , gj ) =

r 

(τνj qiνj )gi

i=1

where

(k)

(k) (k) (k) u(k) τνj qiνj ugi = τνj uqiνj ugi ≤k τνj uSk (gν ,gj )
It follows that f˜ =



cνj

ν,j

r 

r 

(τνj qiνj )gi =

i=1

i=1

where ˜i = H



⎛



⎝

⎞ cνj τνj qiνj ⎠ gi =

ν,j

r 

˜ i gi H

(3.3.12)

i=1

cνj τνj qiνj (1 ≤ i ≤ r)

ν,j

and

(k)

uH˜ u(k) gi
Furthermore, for any l = k + 1, . . . , p, we have + ! ", (l) (l) ordl (uH˜ u(l) ) ≤ maxν,j ordl τνj uH˜ u(l) g g i i i i       (k) (k) uνj (l) uνj (l) ≤ maxν,j max ordl τνj (k) ugν , ordl τνj (k) ugj u gν u gj       u (l) u (l) = maxν,j max ordl , ordl u u (k) gν (k) gj u gν u gj (k)

(l)

≤ ordl u = ordl uf˜ ≤ ordl uf˜ , so that representation (3.3.12) satisfies the condition (l)

(l)

ordl (uH˜ u(l) gi ) ≤ ordl uf˜

(3.3.13)

i

for i = 1, . . . , r. Substituting (3.3.12) into (3.3.9) we obtain f=

r 

˜ i gi + H

i=1



(k)

(Hi − di uHi )gi +

(k) (k) uH ugi =u i



Hi gi

(3.3.14)

(k) (k) uH ugi
(k)

where, denoting each Hi −di uHi in the second sum by Hi , we have the following conditions: (k) (k)

(i) uH˜ ugi
(k) (k)

(ii) uH
ugi
(k) (k)

(iii) uHi ugi
3.3. MULTIVARIABLE DIMENSION POLYNOMIALS

177

Also, for every l = k + 1, . . . , p, the inequality (3.3.13) implies that (l) (l) (l) ordl (uH˜ ugi ) ≤ ordl uf˜ . Therefore, i

(l)

(k)

(l)

(l)

(l) (l) ordl (uH˜ u(l) gi ) ≤ max{ordl (uHi ugi )} ≤ max{ordl (uHi ugi )} ≤ ordl uf i

where the maxima are taken over the set of all indices i that appear in the first sum in (3.3.9). Furthermore, inequality (3.3.8) implies that for every index i in the second sum in (3.3.14), one has (l)

(k)

(l)

(l) ordl (uH
u(l) gi ) ≤ ordl (uHi ugi ) ≤ ordl uf

(l = k + 1, . . . , p)

i

and for every index i in the third sum in (3.3.14) we have (l)

(l)

ordl (uHi u(l) gi ) ≤ ordl uf

(l = k + 1, . . . , p).

Thus, (3.3.14) is a representation of f in the form (3.3.6) with conditions (3.3.7) r  ˜  gi (combining the sums H and (3.3.8) such that if one writes (3.3.14) as f = i (k)

(k)

1

r

i=1

(k) in (3.3.14)), then max{uH˜
u(k) g1 , . . . , uH ˜
ugr }
the types (3.3.7) and (3.3.8). We have arrived at a contradiction with our choice of representation (3.3.6) of f with conditions (3.3.7), (3.3.8) and the smallest (k) (k) (with respect to
every element f ∈ N can be represented in the form (3.3.3) with conditions (3.3.4) and (3.3.5).  The last theorem allows one to construct a Gr¨obner basis of a D-module obner basis of N with N ⊆ E with respect to <1 , . . . ,

CHAPTER 3. DIFFERENCE MODULES

178

(1)

Vr1 ...rp = {u ∈ T e|ordi u ≤ ri for i = 1, . . . , p, and u = τ ug for any τ ∈ T, g ∈ G}, Wr1 ...rp = {u ∈ T e \ Vr1 ...rp |ordi u ≤ ri for i = 1, . . . , p and for every τ ∈ T, (1) (i) g ∈ G such that u = τ
ug , there exists i ∈ {2, . . . , p} such that ordi τ ug > ri }, and Ur1 ...rp = Vr1 ...rp Wr1 ...rp . Then for any (r1 , . . . , rp ) ∈ Np , the set π(Ur1 ...rp ) is a basis of the vector K-space Mr1 ...rp . PROOF. Let us prove, first, that every element τ fi (1 ≤ i ≤ m, τ ∈ T (r1 , . . . , rp )), which does not belong to π(Ur1 ...rp ), can be written as a finite linear combination of elements of π(Ur1 ...rp ) with coefficients in K (so that the set π(Ur1 ...rp ) generates the vector K-space Mr1 ...rp ). Indeed, since (1) τ fi ∈ / π(Ur1 ...rp ), τ ei ∈ / Ur1 ...rp whence τ ei = τ  ugj for some τ  ∈ T , 1 ≤ j ≤ d, (ν) such that ordν (τ  ugj ) ≤ rν (ν = 2, . . . , p). (1)

Let us consider the element gj = aj ugj +. . . (aj ∈ K, aj = 0), where dots are placed instead of the sum of the other terms of gj with nonzero coefficients (ob(1) viously, those terms are less than ugj with respect to the order <1 ). Since gj ∈ (1) (1) N = Ker π, π(gj ) = aj π(ugj ) + · · · = 0, whence π(τ  gj ) = aj π(τ  ugj ) + · · · = aj π(τ ei )+· · · = aj τ fi +· · · = 0, so that τ fi is a finite linear combination with coefficients in K of some elements τ˜l fl (1 ≤ l ≤ m) such that τ˜l ∈ T (r1 , . . . , rp ) and (1) τ˜l el <1 τ  ugj . (ord1 τ˜l ≤ r1 , since τ˜l el <1 τ ei and τ ∈ T (r1 , . . . , rp ); ordν τ˜l ≤ rν (ν) (ν) (ν) (ν = 2, . . . , p), because τ˜l el ≤ν uτ
gj = τ  ugj and ordν (τ  ugj ) ≤ rν .) Thus, we can apply the induction on τ ej (τ ∈ T, 1 ≤ j ≤ m) with respect to the order <1 and obtain that every element τ fi (τ ∈ T (r1 , . . . , rp ), 1 ≤ j ≤ m) can be written as a finite linear combination of elements of π(Ur1 ...rp ) with coefficients in the field K. Now, let us prove that the set π(Ur1 ...rp ) is linearly independent over K. k  ai π(ui ) = 0 for some u1 , . . . , uk ∈ Ur1 ...rp , a1 , . . . , ak ∈ K. Suppose that i=1

Then h =

k 

ai ui is an element of N which is (<1 , . . . ,

i=1

to G. Indeed, if a term u = τ ej appears in h (so that u = ui for some i = (1) (1) 1, . . . , k), then either u is not a multiple of any ugν (1 ≤ ν ≤ d) or u = τ ugν (µ) (µ) for some τ ∈ T, 1 ≤ ν ≤ d, such that ordµ (τ ugν ) > rµ ≥ ordµ uh for some µ, 2 ≤ µ ≤ p. By Proposition 3.3.10, h = 0, whence a1 = · · · = ak = 0. This completes the proof of the theorem.  Now we are ready to prove the main result of this section, the theorem on a multivariable dimension polynomial associated with a difference vector space with an excellent p-dimensional filtration. Theorem 3.3.16 Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let D be the ring of difference operators over K equipped with the standard

3.3. MULTIVARIABLE DIMENSION POLYNOMIALS

179

p-dimensional filtration corresponding to partition (3.3.1) of the set σ. Furthermore, let ni = Card σi (i = 1, . . . , p) and let {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp } be an excellent p-dimensional filtration of a vector σ-K-space M . Then there exists a polynomial φ(t1 , . . . , tp ) ∈ Q[t1 , . . . , tp ] such that (i) φ(r1 , . . . , rp ) = dimK Mr1 ...rp for all sufficiently large (r1 , . . . , rp ) ∈ Zp ; (ii) degti φ ≤ ni (1 ≤ i ≤ p), so that deg φ ≤ n and the polynomial φ(t1 , . . . , tp ) can be represented as φ(t1 , . . . , tp ) =

n1  i1 =0

...

np  ip =0

ai1 ...ip

  t1 + i1 tp + ip ... i1 ip

where ai1 ...ip ∈ Z for all i1 , . . . , ip . PROOF. Since the p-dimensional filtration {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp } is excellent, there exists an element (h1 , . . . , hp ) ∈ Zp such that Dr1 ,...,rp Mh1 ,...,hp = Mr1 +h1 ,...,rp +hp for any (r1 , . . . , rp ) ∈ Np . Furthermore, Mh1 ,...,hp is a finitedimensional vector K-space and any its basis generates M as a left D-module, m  Dyi for some elements y1 , . . . , ym ∈ Mh1 ,...,hp . so M = i=1

Let E be a free D-module with a basis e1 , . . . , em , let N be the kernel of the natural σ-epimorphism π : E → M (π(ei ) = yi for i = 1, . . . , m), and let the set Ur1 ...rp (r1 , . . . , rp ∈ N) be the same as in the conditions of Theorem obner basis of N with respect 3.3.15. Furthermore, let G = {g1 , . . . , gd } be a Gr¨ to <1 , . . . ,

Let Ur 1 ,...,rp = {w ∈ Ur1 ,...,rp |w is not a multiple of any element ugi (1 ≤ i ≤ d)} and let Ur1 ,...,rp = {w ∈ Ur1 ,...,rp | there exists gj ∈ G and τ ∈ T (1)

(ν)

such that w = τ ug
j and ordν (τ ugj ) >  rν for some ν, 2 ≤ ν ≤ p}. Then Ur1 ,...,rp = Ur 1 ,...,rp Ur1 ,...,rp and Ur 1 ,...,rp Ur1 ,...,rp = ∅, whence Card Ur1 ,...,rp = Card Ur 1 ,...,rp + Card Ur1 ,...,rp . By Theorem 1.5.2, there exists a numerical polynomial ω(t1 , . . . , tp ) in p variables t1 , . . . tp such that ω(r1 , . . . , rp ) = Card Ur 1 ,...,rp for all sufficiently large (r1 , . . . , rp ) ∈ Np . In order to express Card Ur1 ,...,rp in terms of r1 , . . . , rp , let us set aij = (1)

(i)

ordi ugj and bij = ordi ugj for i = 1, . . . , p; j = 1, . . . , d. Clearly, a1j = b1j and aij ≤ bij for i = 1, . . . , p; j = 1, . . . , d. Furthermore, for any µ = 1, . . . , p and for any integers k1 , . . . , kµ such that 2 ≤ k1 < · · · < kµ ≤ p, let (1) Vj;k1 ,...,kµ (r1 , . . . , rp ) = {τ ugj |ordi τ ≤ ri − aij for i = 1, . . . , p and ordν τ > rν − bνj if and only if ν is equal to one of the numbers k1 , . . . , kµ }.

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Then Card Vj;k1 ,...,kµ (r1 , . . . , rp ) = φj;k1 ,...,kµ (r1 , . . . , rp ), where φj;k1 ,...,kµ (t1 , . . . , tp ) is a numerical polynomial in p variables t1 , . . . , tp defined by the formula φj;k1 ,...,kµ (t1 , . . . , tp )  

tk1 −1 + nk1 −1 − bk1 −1,j t1 + n1 − b1j ... = n1 nk1 −1    tk1 + nk1 − bk1 ,j tk1 + nk1 − ak1 ,j − × nk1 nk  1 

tkµ −1 + nkµ −1 − bkµ −1,j tk1 +1 + nk1 +1 − bk1 +1,j ... × nk1 +1 nkµ −1     tkµ + nkµ − bkµ ,j tp + np − bpj tkµ + nkµ − akµ ,j − ... . × nkµ nkµ np (3.3.15) (Statement (iii) of Theorem 1.5.2 shows that Card {τ ∈ T | ord1 τ ≤ r1 , . . . , ordp τ  p  ri + ni for any r1 , . . . , rp ∈ N). Clearly, degti φj;k1 ,...,kµ ≤ ni for ≤ rp } = ni i=1 i = 1, . . . , p. (1) Now, for any j = 1, . . . , d, let Vj (r1 , . . . , rp ) = {τ ugj |ordi τ ≤ ri − aij for i = 1, . . . , p and there exists ν ∈ N, 2 ≤ ν ≤ p, such that ordν τ > rν − bνj }. Then the combinatorial principle of inclusion and exclusion implies that Card Vj (r1 , . . . , rp ) = φj (r1 , . . . , rp ), where φj (t1 , . . . , tp ) is a numerical polynomial in p variables t1 , . . . , tp defined by the formula φj (t1 , . . . , tp ) p  φj;k1 (t1 , . . . , tp ) − = k1 =1

×





φj;k1 ,k2 (t1 , . . . , tp ) + · · · + (−1)µ−1

1≤k1
φj;k1 ,...,kµ (t1 , . . . , tp ) + · · · + (−1)p−1 φj;2,...,p (t1 , . . . , tp ) .

1≤k1 <···
It is easy to see that degti φj (t1 , . . . , tp ) ≤ ni for i = 1, . . . , p. Applying the principle of inclusion and exclusion once again we obtain that Card Ur1 ...rp

= Card

d 

Vj (r1 , . . . , rp ) =

j=1

−



d 

Card Vj (r1 , . . . , rp )

j=1

Card (Vj1 (r1 , . . . , rp )

#

Vj2 (r1 , . . . , rp ))

1≤j1 <j2 ≤d

+ · · · + (−1)d−1 Card

d #

Vjν (r1 , . . . , rp ),

ν=1

so it is sufficient to prove that for any s = 1, . . . , d and  for any indices j1 , . . . , js , 1 ≤ j1 < · · · < js ≤ d, Card (Vj1 (r1 , . . . , rp ) · · · Vjs (r1 , . . . , rp )) =

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φj1 ,...,js (r1 , . . . , rp ), where φj1 ,...,js (t1 , . . . , tp ) is a numerical polynomial in ≤ ni for i = 1, . . . , p. It is p variables t1 , . . . , tp such that degti φj1 ,...,j s  clear that the intersection Vj1 (r1 , . . . , rp ) · · · Vjs (r1 , . . . , rp ) is not empty (1) (1) (therefore, φj1 ,...,js = 0) if and only if the leaders ugj1 , . . . , ugjs contain the same element ei (1 ≤ i ≤ m). Let us consider such an intersection   (1) (1) Vjs (r1 , . . . , rp ), let v(j1 , . . . , js ) = lcm(uj1 , . . . , ujs ), Vj1 (r1 , . . . , rp ) . . .  (1) and let v(j1 , . . . , js ) = γν ugν (1 ≤ ν ≤ s; γν ∈ T ). Then Vj1 (r1 , . . . , rp ) . . .  Vjs (r1 , . . . , rp ) is the set of all terms u = τ v(j1 , . . . , js ) such that ordi u ≤ ri (that is, ordi τ ≤ ri − ordi v(j1 , . . . , js )) for i = 1, . . . , p, and for any l = 1, . . . , s, (ν) there exists at least one index ν ∈ {2, . . . , p} such that ordν (τ γl ugjl ) > rν (i.e., (ν)

(1)

ordν τ > rν − ordν v(j1 , . . . , js ) − ordν ugjl + ordν ugjl ). Denoting ordi v(j1 , . . . , js ) (i)

by cj1 ,...,js (1 ≤ i ≤ p) and applying the principle of inclusion and exclusion s # one more time, we obtain that Card Vjµ (r1 , . . . , rp ) is an alternating sum of µ=1

terms of the form Card W (j1 , . . . , js ; k11 , k12 , . . . , k1q1 , k21 , . . . , ksqs ; r1 , . . . , rp ) where W (j1 , . . . , js ; k11 , k12 , . . . , k1q1 , k21 , . . . , ksqs ; r1 , . . . , rp ) = {τ ∈ T |ordi τ ≤ (i) (k) ri − cj1 ,...,js for i = 1, . . . , p, and for any l = 1, . . . , s, ordk τ > rk − cj1 ,...,js + akjl − bkjl if and only if k = kli for some i = 1, . . . , ql } (q1 , . . . , qs are some positive integers from the set {1, . . . , p} and {kiµ |1 ≤ i ≤ s, 1 ≤ µ ≤ qs } is a family of integers such that 2 ≤ ki1 < ki2 < · · · < kiqi ≤ p for i = 1, . . . , s). Thus, it is sufficient to show that Card W (j1 , . . . , js ; k11 , . . . , ksqs ; r1 , . . . , rp ) j1 ,...,js s = ψkj111,...,j ,...,ksq (r1 , . . . , rp ) where ψk11 ,...,ksq (t1 , . . . , tp ) is a numerical polynos

s

s mial in p variables t1 , . . . , tp such that degi ψkj111,...,j ,...,ksqs ≤ ni (i = 1, . . . , p). But this is almost evident: as in the process of evaluation of the value of Card Vj;k1 ,...,kq (r1 , . . . , rp ) (when we used Theorem 1.5.2 (iii) to obtain formula (3.3.15)), we see that Card W (j1 , . . . , js ; k11 , . . . , ksqs ; r1 , . . . , rp ) is a product 

(ν) rν + nν − cj1 ,...,js − Sν (such a term corresponds to a of terms of the form nν number ν ∈ {1, . . . , p} which is different from all kiµ (1 ≤ i ≤ s, 1 ≤ µ ≤ qs ); also terms of the form Sν is defined as max{bνjl − aνjl |1 ≤ l ≤ s}) and 

  (ν) (ν) rν + nν − cj1 ,...,js rν + nν − cj1 ,...,js − Sν − (such a term appears in nν nν the product if ν = kiµ for some i, µ; if ki1 µ1 , . . . , kie µe are all elements of the set {kiµ |1 ≤ i ≤ s, 1 ≤ µ ≤ qs } that are equal to ν (1 ≤ e ≤ s, 1 ≤ i1 < · · · < ie ≤ s), then Sν is defined as min{bνjiλ − aνjiλ |1 ≤ λ ≤ l}). The corresponding s numerical polynomial ψkj111,...,j ,...,ksqs (t1 , . . . , tp ) is a product of p “elementary” nu

(ν) tν + nν − cj1 ,...,js − Sν merical polynomials, each of which is equal to either nν    (ν) (ν)  tν + nν − cj1 ,...,js − Sν tν + nν − cj1 ,...,js − (1 ≤ ν ≤ p). or nν mν

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182

Since the degree of such a product with respect to any variable ti (1 ≤ i ≤ p)  does not exceed ni , this completes the proof of the theorem. Definition 3.3.17 The polynomial φ(t1 , . . . , tp ), whose existence is established by Theorem 3.3.16, is called a (σ1 , . . . , σp )-dimension (or simply dimension) polynomial of the σ-K-vector space M associated with the p-dimensional filtration {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp }. Example 3.3.18 With the notation of Theorem 3.3.16, let n = 2, σ1 = {α1 }, σ2 = {α2 }, and let a σ-K-module M be generated by one element x that satisfies the defining equation p 

ai α1i x

i=0

+

q 

bj α2j x = 0.

j=1

where p, q ≥ 1, ai , bj ∈ K (1 ≤ i ≤ p, 1 ≤ j ≤ q), ap = 0, bq = 0. In other words, M is a factor module of ⎛ a free D-module E⎞= De with a free generator p q   ai α1i + bj α2j ⎠ e. e by its D-submodule N = D ⎝ i=0

j=1

p It is easy to see that the set consisting of a single element g = ( i=0 ai α1i + j obner basis of N with respects to the orders <1 , <2 . In this j=1 bj α2 )e is a Gr¨ case, the proof of Theorem 3.3.15 shows that ⎛ the (σ1 , σ2 )-dimension polynomial ⎞ r  s  of M associated with the natural bifiltration ⎝Mrs = Kα1i α2j x⎠ is q

i=0 j=0

as follows: φ(t1 , t2 ) =

r,s∈N

      t1 + 1 − p t2 + 1 t1 + 1 − p t1 + 1 t2 + 1 − + 1 1 1 1 1    t2 + 1 − q t2 + 1 − = qt1 + pt2 + p + q − pq. × 1 1

(With the notation of the proof, the polynomial  in the first brackets gives

  t2 − 1 t2 + 1 t1   − gives Card Urs Card Urs while the polynomial for 1 1 1 all sufficiently large (r, s) ∈ N2 .) Example 3.3.19 Using the notation of Theorem 3.3.16 once again, let n = 2, σ1 = {α1 }, σ2 = {α2 }, and let a vector σ-K-space M be generated by two elements f1 and f2 with the system of defining equations  α12 f1 + α2 f2 = 0, α1 α2 f1 + α2 f2 = 0. Let E be the free left D-module with free generators e1 , e2 and let N be the D-submodule of E generated by the elements g1 = α12 e1 + α2 e2 , g2 =

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183

∼ E/N and {g1 , g2 } is a Gr¨ obner basis of N with respect α1 α2 e1 +α2 e2 . Then M = to the order <2 (the 2-leaders of g1 and g2 are the terms α2 e2 and α1 α2 e1 , respectively). Since S1 (g1 , g2 ) = α2 g1 − α1 g2 = α22 e2 − α1 α2 e2 is (<1 , <2 )obner basis reduced with respect to {g1 , g2 }, the method of construction of a Gr¨ of N with respect to the orders <1 , <2 justified in Theorem 3.3.13 (see also Exercise 3.3.14) requires that we extend {g1 , g2 } to the set {g1 , g2 , g3 } where g3 = −S1 (g1 , g2 ) = α1 α2 g2 − α22 e2 . Since S1 (gi , g3 ) = 0 (i = 1, 2), Theorem 3.3.13 shows that G is a Gr¨ obner basis of N with respect to <1 , <2 . Let φ(t1 , t2 ) be the dimension polynomial of the D-module M which ⎛ ⎞ corr  s  2  responds to the excellent bifiltration ⎝Mr1 r2 = Kα1i α2j fi ⎠ i=0 j=0 i=1

r1 ,r2 ∈N

associated with the generators f1 and f2 . With the notation of Theorem (1) 3.3.15, Vr1 r2 = {θe1 | θe1 = α1i α2j e1 is not a multiple of ug1 = α12 e1 or
 (1) (1) ug2 = α1 α2 e1 } {τ e2 | τ  e2 = α1k α2l e2 is not a multiple of ug3 = α1 α2 e2 }. Applying Theorem 1.5.7 we obtain that Card Vr1 r2 = 2r1 + r1 + 2 for all sufficiently large r1 , r2 . The corresponding set Wr1 r2 consists of the terms α1r1 α2 e2 , α1r1 α22 e2 . . . , α1r1 α2r2 e2 , α1r1 −1 α2 e2 , so Card Wr1 r2 = r2 + 1. Thus, φ(t1 , t2 ) = 2t1 + 2t2 + 3. Let K be a difference field and let partition (3.3.1) of its basic set σ be fixed. As we have seen, if M is a finitely generated vector σ-K-space, then every finite set of generators Σ of M over the ring of σ-operators D produces an excellent p-dimensional filtration of M and therefore a dimension polynomial of M associated with this filtration. Generally speaking, different finite systems of generators of M over D produce different (σ1 , . . . , σp )-dimension polynomials (we leave the correspondent example to the reader as an exercise), however every dimension polynomial carries certain integers that do not depend on the system of generators. These integers, that characterize the vector σ-K-space M , are called invariants of a dimension polynomial. In what follows, we describe some of the invariants. For any permutation (j1 , . . . , jp ) of the set {1, . . . , p}, we define the lexicographic order <j1 ,...,jp on Np as follows: (r1 , . . . , rp ) <j1 ,...,jp (s1 , . . . , sp ) if and only if either rj1 < sj1 or there exists k ∈ N, 1 ≤ k ≤ p − 1, such that rjν = sjν for ν = 1, . . . , k and rjk+1 < sjk+1 . Now, if Σ ⊆ Np , then Σ will denote the set {e ∈ Σ|e is a maximal element of Σ with respect to one of the p! lexicographic orders <j1 ,...,jp }. Example 3.3.20 Let Σ = {(3, 0, 2), (2, 1, 1), (0, 1, 4), (1, 0, 3), (1, 1, 6), (3, 1, 0), (1, 2, 0)} ⊆ N3 . Then Σ = {(3, 0, 2), (3, 1, 0), (1, 1, 6), (1, 2, 0)}. The following result provides some invariants of dimension polynomial associated with a multi-dimensional filtration.

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184

Theorem 3.3.21 Let K be a difference field with a basic set σ = {α1 , . . . , αn } whose partition (3.3.1) is fixed. Let M be a finitely generated vector σ-K-space, {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp } an excellent p-dimensional filtration of M , and φ(t1 , . . . , tp ) =

n1  i1 =0

...

np  ip =0

ai1 ...ip

  t1 + i1 tp + ip ... i1 ip

the dimension polynomial associated with this filtration. Let Σφ = {(i1 , . . . , ip ) ∈ Np | 0 ≤ ik ≤ nk (k = 1, . . . , p) and ai1 ...ip = 0}. Then d = deg φ, an1 ...np , elements (k1 , . . . , kp ) ∈ Σφ , the corresponding coefficients ak1 ...kp , and the coefficients of the terms of total degree d do not depend on the choice of an excellent filtration.  PROOF. Let {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp } and {Mr 1 ...rp |(r1 , . . . , rp ) ∈ Zp } be two excellent p-dimensional filtrations of the same finitely generated σ-Kvector space M and let φ(t1 , . . . , tp ) and φ (t1 , . . . , tp ) be dimension polynomials associated with these excellent filtrations, respectively. Then there exists an element (s1 , . . . , sp ) ∈ Np such that Mr1 ...rp ⊆ Mr 1 +s1 ,...,rp +sp and Mr 1 ...rp ⊆ Mr1 +s1 ,...,rp +sp for all sufficiently large (r1 , . . . , rp ) ∈ Zp . It follows that there exist u1 , . . . , up ∈ Z such that

and

φ(t1 , . . . , tp ) ≤ φ (t1 + s1 , . . . , tp + sp )

(3.3.16)

φ (t1 , . . . , tp ) ≤ φ(t1 + s1 , . . . , tp + sp )

(3.3.17)

for all integer (and real) values of t1 , . . . , tp such that t1 ≥ u1 , . . . , tp ≥ up . If we set ti = t (1 ≤ i ≤ p) in (3.3.16) and (3.3.17) and let t → ∞ we obtain that φ(t1 , . . . , tp ) and φ (t1 , . . . , tp ) have the same degree d and the same coefficient n of the monomial tn1 1 . . . tp p .  If (k1 , . . . , kp ) ∈ Σφ is the maximal element of Σφ with respect to the lexicographic order ≤j1 ,...,jp , then we set tjp = t, tjp−1 = 2tjp = 2t ,. . . , tj1 = 2tj2 and let t → ∞ in (3.3.16) and (3.3.17). We obtain that (k1 , . . . , kp ) is the maximal k element of Σφ
with respect to ≤j1 ,...,jp and the coefficients of tk11 . . . tpp in the  polynomials φ(t1 , . . . , tp ) and φ (t1 , . . . , tp ) are equal. Finally, let us order the terms of the total degree d in φ and φ using the lexicographic order ≤p,p−1,...,1 , set w1 = t, w2 = 2w1 = 2t , . . . , wp = 2wp−1 , T = 2wp , ti = wi T (1 ≤ i ≤ p) and let t → ∞. Then the inequalities (3.3.16) and (3.3.17) immediately imply that the polynomials φ(t1 , . . . , tp ) and φ (t1 , . . . , tp ) have the same coefficients of the terms of total degree d.  Exercise 3.3.22 With the notation of the last theorem, prove that an1 ...np = σdimK M . Exercise 3.3.23 Let K be a difference field with a basic set σ = {α1 , . . . , αn } (n ≥ 2) and let a partition of σ into two disjoint subsets σ1 = {α1 , . . . , αm } and σ2 = {αm+1 , . . . , αn } be fixed (1 ≤ m < n). Let D, D and D be rings of

3.4. INVERSIVE DIFFERENCE MODULES

185

σ-, σ1 -, and σ2 -operators over K, respectively, considered as filtered rings with standard filtrations (Dr )r∈Z , (Dr )r∈Z , and (Dr )r∈Z . Furthermore, let M be a finitely generated D-module with generators g1 , . . . , gk and for any r ∈ Z, let k   Mr = D Dr gi . i=1

Mimic the proofs of Theorems 3.2.3, 3.2.9, 3.2.11, and 3.2.12 to show that there exists a numerical polynomial λ(t) in one variable t with the following properties: (i) λ(r) = σ2 -dimK Mr for all sufficiently large r ∈ Z (ii) deg λ(t) ≤ m, and the polynomial λ(t) can be written as λ(t) = 

m  t+i where a0 , . . . , am ∈ Z. ai i i=0 (iii) am = σ-dimK M .

3.4

Inversive Difference Modules

Let R be an inversive difference ring with a basic set of automorphisms σ = {α1 , . . . , αn } and let Γ (or Γσ , if one needs to specify the basic set) denote the free commutative group generated by the set σ. As before, we set σ ∗ = {α1 , . . . , αn , α1−1 , . . . , αn−1 } and call R a σ ∗ -ring. n If γ = α1k1 . . . αnkn ∈ Γ, then the number i=1 |ki | is called the order of the element γ, it is denoted by ord γ. For any r ∈ N, the set {γ ∈ Γ|ord γ ≤ r} is denoted by Γ(r).  Definition 3.4.1 An expression of the form γ∈Γ aγ γ, where aγ ∈ R for any γ ∈ Γ and only finitely many elements aγ are different from 0,  is called an ∗ ∗ -) operator over R. Two σ -operators inversive difference (or σ γ∈Γ aγ γ and  γ∈Γ bγ γ are considered to be equal if and only if aγ = bγ for any γ ∈ Γ. ∗ -ring R can be The set of all inversive difference operators naturally  over a σ  b γ = equippedwith a ring  structure if one sets γ∈Γ aγ γ + γ γ∈Γ (aγ +  γ∈Γ a γ)γ = a (γγ ), and γ1 a = bγ )γ, a γ∈Γ aγ γ = γ∈Γ (aa γ )γ, ( γ 1 γ 1 γ∈Γ γ∈Γ  b γ and for any a ∈ R, γ ∈ Γ, γ1 (a)γ1 for any σ ∗ -operators γ∈Γ aγ γ, γ 1 γ∈Γ and extends the multiplication by distributivity. The resulting ring is called the ring of inversive difference (or σ ∗ -) operators over R and denoted by E. It is easy to see that the ring of difference (σ-) operators D introduced in the preceding section is asubring of E. If A = γ∈Γ aγ γ ∈ E, then the number ord A = max{ord γ|aγ = 0} is called the order of the σ ∗ -operator A. For any r ∈ N, the set of all σ ∗ -operators whose order does not exceed r will be denoted by Er . Furthermore, we set Er = 0 if r ∈ Z, r < 0. It is easy to see that the ring E can be considered as a filtered ring with the ascending filtration (Er )r∈Z called a standard filtration of the ring E. Below, while considering E as a filtered ring, we always mean this filtration.

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186

Theorem 3.4.2 Let R be an inversive difference ring with a basic set of automorphisms σ = {α1 , . . . , αn } and let E be the ring of σ ∗ -operators over R. Then the ring E is left Noetherian. PROOF. Let I be a left ideal of E and let D denote the ring of σ-operators over R considered in Section 3.1 (as we have noticed, D is a subring of the  ring E). By Lemma 3.2.7, the ring D is left Noetherian, so that its left ideal I D has a finite system of generators {w1 , . . . , wm }. Now, let u ∈ E. Then there exists an element γ ∈ Tσ ⊆ Γσ such that m   vi wi for some v1 , . . . , vm ∈ D. It follows that, γu ∈ I D, so that γu = u=

m 

i=1

(γ −1 vi )wi , so that w1 , . . . , wm generate the ideal I. Thus, the ring E is

i=1

left Noetherian.



Definition 3.4.3 Let R be an inversive difference ring with a basic set σ = {α1 , . . . , αn } and let E be the ring of inversive difference operators over R. Then a left E-module is said to be an inversive difference R-module (or a σ ∗ -Rmodule). In other words, an R-module M is called a σ ∗ -R-module if elements of the set σ ∗ act on M in such a way that the following conditions hold: (i) (ii) (iii) (iv)

α(x + y) = αx + αy ; α(βx) = β(αx) ; α(ax) = α(a)α(x) ; α(α−1 x) = x

for any α, β ∈ σ ∗ ; x, y ∈ M ; a ∈ R. If R is a σ ∗ -field, then a σ ∗ -R-module M is said to be a vector σ ∗ -R-space (or an inversive difference vector space over R). It is clear that any σ ∗ -R-module can be also treated as a difference module, that is, as a σ-R-module. Furthermore, if M and N are two σ ∗ -R-modules, then any difference (σ-) homomorphism f : M → N has the property that f (αx) = αf (x) for any x ∈ M and α ∈ σ ∗ . Let R be an inversive difference ring with a basic set σ = {α1 , . . . , αn }, and ∗ HomR (M, N ) let M and  N be two σ -R-modules. Then each of the R-modules and M R N can be equipped with a structure of a σ ∗ -R-module if for any k   yi ∈ M R N (x1 , . . . , xk ∈ M ; y1 , . . . , yk ∈ f ∈ HomR (M, N ), i=1 xi N ), and α ∈ σ ∗ , one defines α(f ) (also denoted as αf ) by the formula k k   (α(f ))x = α(f (α−1 x)) and sets α( i=1 xi yi ) = αyi . It is i=1 αxi easy to check that αf ∈ HomR (M, N ) and the action of elements of σ ∗ on HomR (M, N ) satisfies conditions (i) - (iv) of Definition 3.4.3. Furtherk k k    (α(a)αxi αyi ) = more, α(a i=1 xi yi ) = α( i=1 axi yi ) = i=1 k    k α(a)α( i=1 xi yi ) for any i=1 xi yi ∈ M R N , a ∈ R, α ∈ σ ∗ , and = z, α(z1 + z2 ) = αz1 + αz2 , α(βz) = β(αz) for anyα, β ∈ σ ∗ and also α(α−1 z)  z, z1 , z2 ∈ M R N . Thus, the action of elements of σ ∗ on M R N satisfies

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187

conditions (i) - (iv) of Definition 3.4.3 as well. In what follows, while considering  HomR (M, N ) and M R N as σ ∗ -R-modules, we always mean the foregoing inversive differential structures of these modules. Lemma 3.4.4 Let R be an inversive difference (σ ∗ -) ring and let M , N , and P be three σ ∗ -R-modules. Then the canonical mapping M, N ) → HomR (P, HomR (M, N )) η : HomR (P R

  (defined by [(η(f ))x](y) = f (x y) for any f ∈ HomR (P R M, N ), x ∈ P , y ∈ M ) is a σ-isomorphism of σ ∗ -R-modules. PROOF. The fact that η is an isomorphism of R-modules is well-known (see,  (P R M, N ), x ∈ P , y ∈ M, for example [176, Theorem 3.4.3]). If f ∈ HomR and α ∈  σ, then (η(α(f ))(x))(y) = (α(f ))(x y) = α(f (α−1 (x y))) = α(f (α−1 x α−1 y)) = α(η(f )(α−1 x))(α−1 y) = (α(η(f )(α−1 x)))(y) = ((αη)(f )) (x))(y). Thus, η is a σ-isomorphism.  Let R be an inversive difference ring with a basic set σ and E the ring of σ ∗ -operators over R. For any σ ∗ -R-module M , the set C(M ) = {x ∈ M |αx = x for all α ∈ σ} is called the set of constants of the module M , elements of this set are called constants. It is easy to see that C(M ) is a subgroup of the additive group of M and the mapping C : M → C(M ) is a functor from the category of σ ∗ -R-modules (i. e., the category of all left E-modules) to the category of Abelian groups. Lemma 3.4.5 Let R be an inversive difference (σ ∗ -) ring and E the ring of σ ∗ -operators over R. Then (i) C(HomR (M, N )) = HomE (M, N ) for any two σ ∗ -R-modules M and N . (ii) The functors C and HomE (R, ·) are naturally isomorphic. (In this case we write C  HomE (R, ·) .) (iii) The functor C is left exact and for any positive integer p, its p-th right derived functor is naturally isomorphic to the functor ExtpE (R, ·).  (iv) If M and N are two σ ∗ -R-modules, then HomE (· R M, N )  HomE  (·, HomR (M, N )) and HomE (M R ·, N )  HomE (M, HomR (·, N )). PROOF. The first statement follows from the definition of the action of elements of σ on HomR (M, N ). Indeed, φ ∈ C(HomR (M, N )) if and only if α(φ(α−1 (x)) = φ(x) for every α ∈ σ, x ∈ M , that is equivalent to the inclusion φ ∈ HomE (M, N ). Statement (ii) is a direct consequence of (i) and the obvious fact that the functors C(·) and C(HomR (R, ·) are naturally isomorphic. Since the functor HomE (R, ·) is left exact, statement (ii) implies that C(·) is left exact as well. Now, the natural isomorphism of the functors C(·) and HomE (R, ·) implies the natural isomorphism of their p-th right derived functors p Rp C and Rp HomE (R, ·) = Ext E (R, ·) for any p > 0. (· By Lemma 3.4.4, Hom R R M, N )  HomR (·, HomR (M, N )), whence  C(HomR (· R M, N ))  C(HomR (·, HomR (M, N ))). Applying (i) we obtain

188

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 that HomE (· R M, N )  HomE (·, HomR (M, N )). The statement about the other pair of functors in (iv) can be proved in the same way.  Theorem 3.4.6 Let R be an inversive difference ring with a basic set σ, E the ring of σ ∗ -operators over R, and M , N two σ ∗ -R-modules. Then for any positive integers p and q, there exists a spectral sequence converging to Extp+q E (M, N ) whose second term is equal to E2p,q = (Rp C)(ExtqR (M, N )). PROOF. Because of the statement of Theorem 1.1.13, it is sufficient to prove the following fact: Let N be an injective E-module and p a positive integer. Then (Rp C)(HomR (M, N )) = 0 for any E-module M . First, let us prove the last equality for an  E-module M which is flat as an R-module. In this case the functor HomE (· R M, N ) is exact, hence the functor HomE (·, HomR (M, N )) is also exact (by Lemma 3.4.5 (iv) these two functors are naturally isomorphic). It follows that HomR (M, N ) is an injective E-module, hence ExtpE (R, HomR (M, N )) = 0 for all p > 0. Applying Lemma 3.4.5 (iii) we obtain that (Rp C)(HomR (M, N )) = 0 for all p > 0. Now, let M be arbitrary E-module and let F : · · · → F1 → F0 → M → 0 be a flat (e. g., free) resolution of M as an R-module. (Each Fi is a flat R-module and the mappings are homomorphisms  of R-modules.) (E By Lemma 3.4.5 (iv), Hom E R ·, N )  HomE (E, HomR (·, N )), hence  HomE (E R ·, N ) HomR (·, N ). Since the E-module N is injective, the ·, N ) is exact, hence HomR (·, N ) is also exact. Applying functor HomE (E functor C to the injective resolution 0 → HomR (M, N ) → HomR (F, N ) of the E-module HomR (M, N ) we obtain that (Rp C)(HomR (M, N )) = H p (C(HomR (F, N ))) is isomorphic to H p (HomE (F, N )) for every p > 0. By the first part of the proof, H p (HomE (F, N )) = 0, hence (Rp C)(HomR (M, N )) = 0 for any E-module M and for any p > 0. This completes the proof.  The last theorem finds its applications in the analysis of systems of linear difference equations considered in the rest of this section. Inversive difference modules and systems of linear difference equations Let R be an inversive difference ring with a basis set σ = {α1 , . . . , αn }, σ ∗ = {α1 , . . . , αn , α1−1 , . . . , αn−1 }, Γ the free commutative group generated by σ, and E the ring of σ ∗ -operators over R. For any two σ ∗ -R-modules M and N , let B(M, N ) denote the set of all additive mappings from M to N with the following property. For every β ∈ B(M, N ), there exists γβ ∈ Γ such that β(ax) = γβ (a)β(x) for any a ∈ R, x ∈ M . (The mapping β → γβ is not supposed to be injective  or surjective.) Furthermore, let P(M, N ) denote the set of all formal sums β∈B(M,N ) aβ β, where aβ ∈ R for any β ∈ B(M, N ) and only finitely many elements aβ are different from 0. ∗ It is easy to see that  P(M, N ) becomes a σ -R-module if ∗one defines  α(a )(αβ) for every α ∈ σ . (Clearly, α( β∈B(M,N ) aβ β) = β β∈B(M,N ) αβ ∈ B(M, N ) if β ∈ B(M, N ); in this case γαβ = αγβ .) In what follows,

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189

 we also treat HomR (M, N ) and E R M as left E- (that is, σ ∗ -R-) modules. The corresponding structure of the first module is defined as in the beginning of and the E-module structure on the second one is natural:  this section, ω(ω1 x) = (ωω1 ) x for every ω, ω1 ∈ E, x ∈ M . Lemma 3.4.7 Let M be aσ ∗ -R-module and M ∗ = HomR (M, R). Then the E-modules P(M, R) and E R M ∗ are isomorphic.  PROOF. Consider the mapping φ : E R M ∗ → P(M, R) such that k k  (φ( i=1 ai (ωi e∗i )))(e) = i=1 ai ωi (e∗i (e)) (ai ∈ R, ωi ∈ E, e∗i ∈ M ∗ for i = 1, . . . , k and e ∈ M ). It is easy to see that φ is a σ-homomorphism. To show that φ isbijective, one just  needs to verify mapping sthat the  s βi ) γβ−1 ψ : P(M, R) → E R M ∗ defined by ψ( i=1 ai βi ) = i=1 ai (γβi i  (ai ∈ R, βi ∈ B(M, R) for i = 1, . . . , s) is inverse of φ. Let P = (ωij )1≤i≤s,1≤j≤m be an s × m-matrix over E, and let f1 , . . . , fs be elements of the σ ∗ -ring R. We are going to consider the problem of solvability of a system of linear equations Pu = g (3.4.1) with respect to unknown elements u1 , . . . , um of the ring R (u and g denote the column of the unknowns (u1 , . . . , um )T and the column (g1 , . . . , gs )T , respectively). In what follows we treat the R-modules E = Rm and F = Rs as σ ∗ -R-modules such that α((a1 , . . . , ak )T ) = (α(a1 ), . . . , α(ak ))T for any α ∈ σ ∗ (k = m or k = s, a1 , . . . , ak ∈ R). The ring of s × m-matrices Es×m (with entries from E) will be also treated as a σ ∗ -R-module where α(ωij )1≤i≤s, 1≤j≤m = (α(ωij ))1≤i≤s, 1≤j≤m (α ∈ σ ∗ (ωij )1≤i≤s, 1≤j≤m ∈ Es×m ). Lemma 3.4.8 With the above notation, the E-modules P(E, F ) and Es×m are isomorphic. PROOF. Let Pij denote the matrix from Es×m whose only nonzero entry is 1 at the intersection of the ith row and jth column. Since matrices Pij (1 ≤ i ≤ s, 1 ≤ j ≤ m) generate the E-module Es×m , it is sufficient to define an isomorphism φ : Es×m → P(E, F ) on these matrices. We define φ(Pij ) by its action on elements of E as follows. If e = (c1 , . . . , cm )T ∈ E, then (φ(Pij ))(e) = (0, . . . , cj , . . . , 0)T (the ith coordinate is cj , and all other coordinates are zeros). The inverse mapping ψ : P(E, F ) → Es×m acts on generators β ∈ B(E, F ) of the E-module P(E, F ) as follows: ψ(β) = (γβ (aij )γβ )1≤i≤s,1≤j≤m s where elements aij ∈ R are defined by the relationships γβ−1 β(ek ) = j=1 ajk fj for the R-homomorphism γβ−1 β. (e1 , . . . , em and f1 , . . . , fs are standard bases of E = Rm and F = Rs , respectively.) It is easy to check that φψ and ψφ are  identical mappings of Es×m and P(E, F ), respectively. In the rest of this section we use the notation introduced before  Lemma ∗ = Hom (E, R), F 3.4.8. Furthermore, we consider P(E, F ), E R R P(E, R),   to the action of σ ∗ defined and F R (E R E ∗ ) as σ ∗ -R-modules with respect  ∗ as at the beginning of the section and treat E E as a left E-module with

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190

 ∗  ∗   ∗ ∗ the natural structure  e ∗ ∈ E , ω ∈ ∗E). In  (ω∗ (ω e ) = ω ω efor any ) is a  generator of F R (E R E ) and α ∈ σ , then particular,  if f (ω e α(f (ω e∗ )) = α(f ) (αω e∗ ). Let us consider the diagram P(E, F )

η



-

ξ

@ I @ @ @ µ @λ @ ν @ @ @@@ R @  F R (E R E ∗ )

F

 R

P(E, R))

 δ

(3.4.2)

where all six mappings are difference homomorphisms defined at the generators as follows: γβ−1 β), ω(e∗ (·)), µ(f β) = f (γβ δ(f (ω e∗ )) = f ν(f

m  ¯ = ¯ i) e∗i ), (γβ¯ β(e (ω e∗ )) = ω(e∗ (·))f, λ(β) i=1

¯ = η(β)

m 

¯ i) β(e



γβ¯ e∗i ,

ξ(f



β) = β(·)f

i=1

for any f ∈ F, ω ∈ E, e∗ ∈ E ∗ , β ∈ B(E, R), β¯ ∈ B(E, F ) ( (ei )1≤i≤m denotes the standard basis of E over R, and (e∗i )1≤i≤m denotes the dual basis of E ∗ ). Lemma 3.4.9 All mappings in diagram (3.4.2) are difference isomorphisms, η = ξ −1 , µ = δ −1 λ = ν −1 , and the diagram is commutative. PROOF. We shall prove that µ = δ −1 and νµ = ξ. (The other required relationships can be proved in a similar way.) Let f ∈ F,  e∗ ∈ E ∗ ,   aγ γ ∈ E, and β ∈ B(E, R). Then (µδ)(f ( γ∈Γ aγ γ e∗ )) = γ∈Γ    −1 ∗    ∗  ∗ (γ γ γe )) = f ( γ∈Γ aγ γ e ) µ(f γ∈Γ aγ γ(e (·))) = γ∈Γ aγ (f    −1   −1 γβ γβ β = f β, so µ = γβ β)) = f and (δµ)(f β) = δ(f (γβ   δ −1 . Furthermore, for any generator f β of the E-module F P(E, R)  R−1  γβ β)) = (f ∈ F, β ∈ B(E, R)), we have (νµ)(f β) = ν(f (γβ  γβ (γβ−1 β(·))f = β(·)f = ξ(f β) that proves the equality νµ = ξ.  Let P ∈ P(E, F ) and let P¯ : P(F, R) → P(E, R) be the homomorphism of  E-modules such that P¯ (β) = βPfor any β ∈ B(E, R). Let φF : E R F ∗ → P(F, R) and ψE : P(E, R) → E R E ∗ be difference  isomorphisms  defined in the proof of Lemma 3.4.7. Let P ∗ = ψE P¯ φF : E R F ∗ → E R E ∗ , N =

3.4. INVERSIVE DIFFERENCE MODULES

191

Ker P ∗ , and M = Coker P ∗ . Applying functor HomE (·, R) to the standard exact sequence of left E-modules i

→E 0→N −



P∗

F ∗ −−→ E

R



j

E∗ − →M →0

(3.4.3)

R

(i and j are the natural injection and projection, respectively), we obtain the exact sequence of σ ∗ -R-modules j∗

0 → HomE (M, R) −→ HomE (E



P ∗∗

E ∗ , R) −−→ HomE (E



R

i∗

F ∗ , R)

R

−→ HomE (N, R)

(3.4.4)

Now let us consider the homomorphism of σ-modules θ : P(E, F ) → HomE (P(F, R), P(E, R)) such that (θ(P ))(P1 ) = P1 P for every P ∈ P(E, F ), P1 ∈ P(F, R) and the exact sequence of σ ∗ -R-modules λ

→F P(E, F ) −



(E



R ρ

− → HomE (E



F ∗, E

R

E∗) − → HomR (F ∗ , E 



R



E∗)

R

E∗) − → HomE (P(F, R), P(E, R)) π

R

where λ is the same as in  diagram other σ-homomorphisms  (3.4.2) and the   are defined as follows: ((f (ω e∗ ))(f ∗ ) = f ∗ (f ) e∗ , (ρ(h))(ω f ∗ ) = ∗ ∗ ∗ ∗ ωh(f  any∗f ∈F, ω∗∈ E, e ∈ E , h ∈ HomR (F ,  ), ∗and π(χ) = φE χψF for E R E ), and χ ∈ HomE (E R F , E R E ). (φE and ψF denote the σhomomorphisms defined in the proof of Lemma 3.4.7.) It is easy to check that −1 is defined by λ, , ρ, andπ are isomorphisms of σ ∗ -R-modules. For example,   s −1 ∗ ∗ ∗  (ζ) = i=1 fi ζ(fi ) for every ζ ∈ HomR (F , E R E ) ((fi )1≤i≤s is the standard basis of F and (fi∗ )1≤i≤s is the dual basis of the σ ∗ -R-module F ∗ ). Lemma 3.4.10 With the above notation, θ = πρλ. PROOF. Clearly, it is sufficient to verify the equality at an element β¯ ∈ B(E, F ). If ω ∈ E and f ∗ ∈ F ∗ , then m   ∗ ∗ ¯ ¯ e ) (ω (ρλ(β))(ω f ) = ρ (γ ¯ f ∗) β(ei ) β

i

i=1

=

m 

¯ i ))γ ¯ ωf ∗ (β(e β



e∗i .

i=1

¯ ¯ F (β) = Now, for any ∈ B(F, R),we have (πρλ( β))(β) = φE (ρλ(β))ψ  β −1 m ∗ ¯ ¯ β) = φ β( β(e ))γ . To complete the proof it γ e φE (ρλ(β))(γ ¯ β E i i i=1 β mβ is sufficient to notice that for every e = k=1 ck ek ∈ E ( e1 , . . . , em is the

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192

m 

¯ i ))γ ¯ β(β(e β



 e∗i

(e) = standard basis of E and c1 , . . . , cm ∈ R), φE i=1  m m   ¯ = (θ(β))(β)(e) ¯ ¯ ¯ whence θ = πρλ.  β β(ci ei ) = β β ci ei = β β(e) i=1

i=1

Let us associate with every mapping P ∈ P(E, F ) a set Com P consisting of all f ∈ F such that for every pair (G = Rt , P1 ∈ P(F, G)) with the condition P1 P = 0, one has P1 f = 0. Clearly, the image Im P of the mapping P is a subset of Com P . The following example shows that the inclusion can be proper. Example 3.4.11 Let R = Q(x) be the field of rational fractions over Q treated as an inversive difference field with one translation α such that (αf )(x) = f (2x) for every f (x) ∈ R. Let E denote the ring of inversive difference operators 2 , F = R2 , and P the element of P(E, F ) defined by the maover R, E = R α−1 0 trix . (By Lemma 3.4.8, every element of P(E, F ) can be defined 0 1

  α−1 0 u1 | u1 , u2 ∈ by a 2 × 2-matrix over E.) Note that Im P = { u2 0 1 

(α − 1)u1 | u1 , u2 ∈ R} is a proper E-submodule of F . It follows R} = { u2 from the fact that 1 cannot be written as (α − 1)u1 with u1 ∈ R: if u1 = an xn + · · · + a1 x + a0 (all coefficients ai , bj belong to R, an = 0, and bm = 0), bm xm + · · · + b1 x + b0 then (α − 1)u1 = h1 (x)/h2 (x) where h1 (x) = (2n − 2m )an bm xm+n + · · · + 2(a1 b0 − a0 b1 )x and h2 (x) = 2m b2m x2m + · · · + 3b1 b0 x + b20 . It is easy to see that h1 (x) = h2 (x).

 v1 ∈ Com P if and only if On the other hand, Com P = F . Indeed, v = v2 for every s =  1, 2, . . . and for every matrix W = (ωij )1≤i≤s,1≤j≤2 , the equality α−1 0 W = 0 implies W v = 0. (In the last two equalities 0 in the right0 1 hand sides denotes the zero s × 2- and s × 1- matrices, respectively.) Since the

 α−1 0 ring E does not have zero divisors, the equality W = 0 implies that 0 1 W is a zero matrix, so W v = 0 for all v ∈ F , whence Com P = F . The following theorem gives a connection between Im P and Com P under the above conventions. The theorem allows to reduce the description of the image of the operator P to the description of Com P using the spectral sequence from Theorem 3.4.6. Theorem 3.4.12 With the above notation, for every P ∈  P(E, F ), there exist isomorphisms of σ ∗ -R-modules ϑE : E → HomE (E R E ∗ , R) and ϑF : F → HomE (E R F ∗ , R) such that the following diagram is commutative.

3.4. INVERSIVE DIFFERENCE MODULES HomE (E

 R

P ∗∗

E ∗ , R)

-

193

HomE (E

6 ϑE

 R

F ∗ , R)

6 ϑF -

P

E

(3.4.5)

F

Furthermore, we have the following properties of the exact sequence (3.4.4). (i) Im j ∗ ∼ = Ker P ; (ii) Ker i∗ ∼ = Com P ; (iii) Com P/Im P ∼ = Ext1E (M, R).

 PROOF. difference homomorphisms φE : E R E ∗ → P(E, R) and  The proof of Lemma 3.4.7 induce hoφF : E R F ∗ → P(F, R) defined in the  and momorphisms  of E-modules φ¯E : HomE (E R E ∗ , R) → HomE (P(E, R)−1 γβ β) φ¯F : HomE (E R F ∗ , R) → HomE (P(F, R) where (φ¯E (g))(β) = g(γβ  for every g ∈ HomE (E R E ∗ , R), β ∈ B(E, R), and φ¯F acts in a similar way. Now one can define the mappings χE : E → HomE (P(E, R), R) and χF : F → HomE (P(F, R), R) by setting (χE (e))(β) = β(e) and (χF (f ))(β1 ) = β1 (f ) for any e ∈ E, f ∈ F, β ∈ P(E, R), and β1 ∈ P(F, R). It is easy to check that χE and χF are difference isomorphisms and the diagram P¯ ∗

HomE (P(E, R), R)

-

HomE (P(F, R), R)

6 χE

6 χF P

E

-

(3.4.6)

F

is commutative (the mapping P¯ ∗ is obtain by applying functor HomE (·, R) to the mapping P¯ : P(F, R) → P(E, R) considered above). The inverse difference isomorphism of χE is the mapping λE = χ−1 E : HomE (P(E, R), R) → E such m  that λE (g) = g(e∗i )ei for any g ∈ HomE (P(E, R), R), and the inverse mapi=1

ping of χF is defined similarly. (As before, (ei )1≤i≤m is the standard basis of the R-module E and (e∗i )1≤i≤m ⊆ HomR (E, R) ⊆ P(E, R) is the corresponding dual basis.) Indeed, for any e ∈ E, (λE χE )(e) =

m  i=1

(χ ¯E (e))(e∗i )ei =

m 

e∗i (e)ei = e,

i=1

hence λE χE is the identity automorphism of E. Also, a routine computation shows that the mapping χE λE leaves fixed every element of HomE (P(E, R), R) ¯∗ (thus, λE = χ−1 E ) and P χE = χF P .

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194

The commutativity of diagram (3.4.6) and Lemma 3.4.7 imply the commutativity of diagram (3.4.5) with ϑE = Hom E (φE , R)χE and ϑF = HomE (φF , R)χF (φE : E R E ∗ → P(E, R) and φF : E R F ∗ → P(F, R) are difference isomorphisms defined in the proof of Lemma 3.4.7). Therefore, Ker P ∼ = Ker P ∗∗ = ∗ Im j = HomE (M, R) (see the exact sequence (3.4.3) where M = Coker P ∗ ). This proves statement (i). To prove (ii) assume first that ζ ∈ Ker i∗ and P P  = 0 for every P  ∈ P(F, G) (G = Rt for some positive integer t). Then P ∗ P ∗ = (P  P )∗ = 0 ∗ hence Im P ∗ ⊆ Ker  P ∗ = Im i and one can consider the well-defined mapping −1 ∗  = i P : E R G → N (as in sequence (3.4.3), N = Ker i). Now, if ζ = ϑF (z) ∈ Ker i∗ (z ∈ F ), then P ∗∗ (ζ) = (i)∗ (ζ) = ∗ i∗ (ζ) = 0 hence (ϑG P  )(ζ) = 0. It follows that P  (z) = 0, so that z ∈ Com P . Thus, Ker i∗ ⊆ ϑF (Com P ). Conversely, let z ∈ Com P and ζ = ϑF (z). Let us fix some x ∈ N and consider the homomorphism of E-modules δ : E → N such that δ(1) = x. The composition of the σ ∗ -R-isomorphisms HomE (E, E



F ∗) → E

R



φF

F ∗ −−→ P(F, R),

R

where the first  mapping is the natural isomorphism, sends the element iδ ∈ P ∈ P(F, R). Denoting the natural isoHomE (E, E R F ∗ ) to some element  morphism of E-modules E → E R R∗ by τ , we obtain that iδ = P ∗ τ . Indeed, dk  ∗ d fk ) where ak ∈ R, ωk = let x = iδ(1) = k=1 ai (ωk l=1 bkl γkl ∈ E (γkl ∈ Γ), and fk∗ ∈ F ∗ (1 ≤ k ≤ d). Then (P ∗ τ )(1) = P ∗ (1 1∗ ) = ψF P¯  φR (1 1∗ ) = ψF P¯  (1∗ (·))  d  d   dk    ∗ ∗ = ψF ak ωk (fk (·)) = ψF ak bkl γkl (fl (·)) k=1

=

d  k=1

ak

dk  l=1

k=1

bkl (γkl



−1 γkl γkl fl∗ =

d 

l=1

ak (ωk



fk∗ = x = iδ(1)

k=1

(ψF is the same as in Lemma 3.4.7), whence iδ = P ∗ τ. Since Im i = Ker P ∗ , P ∗ P ∗ τ = P ∗ iδ = 0 whence P ∗ P ∗ = 0. Applying ∗ we obtain that P  P = 0. Furthermore, since z ∈ Com P , we have P  (z) = 0 and P ∗∗ (ζ) = ζP ∗ = 0. Therefore, ζ(i(x)) = ζ(iδ(1)) = ζ(P ∗ τ (1)) = 0, that is, (i∗ (ζ))(x) = 0. Since x is an arbitrary element of N , i∗ (ζ)) = 0, that is, ζ ∈ Ker i∗ . Thus, ϑF (Com P ) is contained in Ker i∗ whence ϑF (Com P ) = Ker i∗ . In order to prove the last statement of the theorem, let us break the exi act sequence (3.4.3) into two short exact sequences of E-modules, 0 → N − →   q j ε → L → 0 and 0 → L − → E R E∗ − → M → 0 where L = Im P ∗ , q is a E R F∗ − projection, and ε is the embedding. Applying functor HomE (·, R) to these two sequences we obtain the exact sequences

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS q∗

0 → HomE (L, R) −→ HomE (E

 R

195

i∗

F ∗ , R) −→ HomE (N, R) and

 j∗ ε∗ 0 → HomE (M, R) −→ HomE (E R E ∗ , R) −→ HomE (L, R) →  Ext1E (M, R) → Ext1E (E R E ∗ , R) = 0. (The mapping q ∗ identifies HomE (L, R) with the module Ker i∗ = ϑF (Com P ) ∗ ∗ and ϑ−1 F q ε ϑE = P . Thus,  Ext1E (M, R) ∼ = HomE (L, R) / ε∗ (HomE (E R E ∗ , R)) = HomE (L, R) / q ∗ ϑE (E)(q ∗ )−1 ϑF (Com P ) / (q ∗ )−1 ϑF P (E) ∼ = Com P / Im P . 

3.5

σ ∗ -Dimension Polynomials and their Invariants

Let R be an inversive difference ring with a basic set σ and let E be the ring of σ ∗ -operators over R considered as a filtered ring with the filtration (Er )r∈Z introduced in the preceding section. Definition 3.5.1 Let M be a σ ∗ -R-module. An ascending chain (Mr )r∈Z of R-submodules of M is called a filtration of
M if Er Ms ⊆ Mr+s for all r, s ∈ Z, Mr = 0 for all sufficiently small r ∈ Z, and r∈Z Mr = M . A filtration (Mr )r∈Z of the the σ ∗ -R-module M is called excellent if all R-modules Mr (r ∈ Z) are finitely generated and there exists r0 ∈ Z such that Mr = Er−r0 Mr0 for any r ∈ Z, r ≥ r0 . Theorem 3.5.2 Let R be an Artinian σ ∗ -ring with a basic set σ = {α1 , . . . , αn } and let (Mr )r∈Z be an excellent filtration of a σ ∗ -R-module M . Then there exists a numerical polynomial χ(t) in one variable t such that (i) χ(r) = lR (Mr ) for all sufficiently large r ∈ Z ; (ii) deg χ(t) ≤ n and the polynomial χ(t) can be represented in the form χ(t) =

n  i=0

 t+i i

2i ai

(3.5.1)

where a0 , . . . , an ∈ Z. PROOF. Let gr E denote the graded ring associated with the filtration  gr (Er )r∈Z , that is, gr E = s E where gr E = Es /Es−1 for all s ∈ Z. Let s∈Z x1 , . . . , x2n be the images in gr E of the elements α1 , . . . , αn , α1−1 , . . . , αn−1 , respectively (so that xi = αi + E0 ∈ gr1 E = E1 /E0 and xn+i = αi−1 + E0 ∈ gr1 E for i = 1, . . . , n). It is easy to see that the elements x1 , . . . , x2n generate the ring gr E over R, xi xj = xj xi (1 ≤ i, j ≤ 2n,) and xi xn+i = 0 for i = 1, . . . , n. Furthermore, xi a = α(a)xi and xn+i a = α−1 (a)xn+i (1 ≤ i ≤ n). In the rest of the proof the ring gr E will be also denoted by R{x1 , . . . , x2n }. A homogeneous component grs E (s ∈ N) of this graded ring is an R-module

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196

generated by the set of all monomials xki11 . . . xkinn such that k1 , . . . , kn ∈ N,  n iµ − iν = n for any µ, ν ∈ {1, . . . , n}. ν=1 kν = s, and Let gr M = s∈Z grs M be the graded gr E-module associated with the excellent filtration (Mr )r∈Z . (In what follows, the module gr M will be also denoted by M and its homogeneous components grs M = Ms /Ms−1 (s ∈ Z) will be denoted by M (s) ). First of all, repeating the beginning of the proof of Theorem 3.2.3,  we obtain that gr M is a finitely generated gr E-module. Since lR (M (s) ), the theorem will be proved if one can prove the existence lR (Mr ) = s≤r

of a numerical polynomial f (t) in one variable t such that (a) f (s) = lR (M (s) ) for all sufficiently large s ∈ Z; (b) deg f (t) ≤ n − 1 and the polynomial f (t) can be represented as f (t) = 

n−1  t+i i+1 where b0 , . . . , bn−1 ∈ Z. 2 bi i i=0 (Indeed, if such a polynomial f (t) exists, then Corollary 1.4.7 implies the existence of the polynomial χ(t) with the desired properties.) We are going to prove the existence of the polynomial f (t) with the properties (a) and (b) by induction on n = Card σ. If n = 0, then gr M is a finitely generated module over the Artinian ring R. In this case, M (s) = 0 for all sufficiently large s ∈ Z, so that the polynomial f (t) = 0 satisfies conditions (a) and (b). Now, suppose that n > 0. Let us consider the exact sequence of finitely generated R{x1 , . . . , x2n }-modules θ

n 0 → Ker θn → M −→ xn M → 0

(3.5.2)

where θn is the mapping of multiplication by xn , that is, θn (y) = xn y for any y ∈ M. It is easy to see that θn is an additive mapping and θn (ay) = αn (a)θn (y) for any y ∈ M, a ∈ R. By Proposition 3.1.4 (with K = (Ker θn )(s) , M = M (s) , N = (xn M)(s) , and δ = θn ), we obtain that lR ((Ker θn )(s) ) + lR ((xn M)(s) ) = lR (M (s) ) for every s ∈ Z. (In accordance with the above notation, we denote the s-th homogeneous component of a graded module P by P (s) .) Since Ker θn and xn M are annihilated by the elements xn and x2n , respectively, one can consider Ker θn as a graded R{x1 , . . . , xn−1 , xn+1 , . . . , x2n }module and treat xn M as a graded R{x1 , . . . , x2n−1 }-module. (By R{x1 , . . . , xn−1 , xn+1 , . . . , x2n } and R{x1 , . . . , x2n−1 } we denote the graded subrings of the ring R{x1 , . . . , x2n } whose elements can be written in the form that does not contain xn and x2n , respectively. The homogeneous components of each of these subrings are the intersections of the corresponding homogeneous components of R{x1 , . . . , x2n } with the subring.) ¯ ∗ -operators over R Let σ ¯ = {α1 , . . . , αn−1 } and let E¯ denote the ring of σ (when R is treated as an inversive difference ring with the basic set σ ¯ ). Then gr E¯ = R{x1 , . . . , xn−1 , xn+1 , . . . , x2n−1 }. By the induction hypothesis,  for any (p) , finitely generated R{x1 , . . . , xn−1 , xn+1 , . . . , x2n−1 }-module N = p∈Z N

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197

there exists a numerical polynomial fN (t) in one variable t such that fN (t) = lR (N (p) ) for all sufficiently large p ∈ Z. Furthermore, deg fN (t) ≤ n − 2 and 

n−2  t+i i+1 for some 2 ci the polynomial fN (t) can be written as fN (t) = i i=0 ¯ c0 , . . . , cn−2 ∈ Z. (We(q)assume that n ≥ 2. If n = 1, then E = R and fN (t) = 0). If L = is a finitely generated graded module over the ring q∈Z L R{x1 , . . . , xn−1 , xn+1 , . . . , x2n }, then the first and the last terms of the exact sequence of R-modules 2n 0 → (Ker θ2n )(q) → L(q) −− → L(q+1) → L(q+1) /x2n L(q) → 0

θ

(3.5.3)

(θ2n is the mapping of multiplication by x2n ) are homogeneous components of finitely generated graded R{x1 , . . . , xn−1 , xn+1 , . . . , x2n−1 }-modules. Breaking the sequence (3.5.3) into two short exact sequences and applying Proposition 3.1.4 (with δ = θ2n ) we obtain that lR (L(q+1) ) − lR (L(q) ) = lR (L(q+1) /x2n L(q) ) − lR ((Ker θ2n )(q) ). By the induction hypothesis, there exists a numerical polynomial gL (t) = 

t+i (c0 , . . . , cn−2 ∈ Z) such that gL (s) = lR (L(q+1) ) − lR (L(q) ) 2i+1 ci i i=0 for all sufficiently large q ∈ Z, say, for all q ≥ q0 for some integer q0 . Since

n−2 

lR (L(q) ) = lR (L(q0 ) ) +

q  

lR (L(s) ) − lR (L(s−1) ) ,

s=q0 +1

Corollary 1.4.7 implies that there exists a numerical polynomial hL (t) such that hL (q) = lR (L(q) ) for all sufficiently large q ∈ Z and the polynomial hL (t) can 

n−1  i  t + i where c0 , . . . , cn−1 ∈ Z and ci = ci−1 for 2 ci be written as hL (t) = i i=0 i = 1, . . . , n − 1. Clearly, statement is true for any finitely generated  a similar graded module K = q∈Z K (q) over the ring R{x1 , . . . , x2n−1 }. Applying the above reasoning to the graded modules of the exact sequence 

n−1  t+i (3.5.2) we obtain that there exist numerical polynomials f1 (t) = 2i bi i i=0 

n−1  t+i (bi , bi ∈ Z for i = 0, . . . , n − 1) such that and f2 (t) = 2i bi i i=0 f1 (s) = lR ((Ker θn )(s) ) and f2 (s) = lR ((xn M)(s) ) for all sufficiently large s ∈ Z. Furthermore, the exact sequence (3.5.2) shows that lR (M (s) ) = lR (Ker θn )(s) )+ lR ((xn M)(s) ) for all sufficiently large s ∈ Z. Now, let us show that lR ((xi M)(s) ) = lR ((xn+i M)(s) ) for  any s ∈ Z, i = 1, . . . , n. Indeed, lR ((xn+i M)(s) ) = lR (αi−1 Ms + Ms )/Ms = lR (αi−1 Ms + Ms ) − lR (Ms ). Applying Proposition 3.1.4 (where δ is the bijective mapping of

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multiplication by αi ) we obtain that lR (αi−1 Ms + Ms ) = lR (αi (αi−1 Ms + Ms )) = lR (αi Ms + Ms ) whence lR ((xn+i M)(s) ) = lR (αi Ms + Ms /Ms ) = lR ((xi M)(s) ). Thus, lR (M (s) ) = lR (Ker θn )(s) ) + lR ((xn M)(s) ) = lR (Ker θn )(s) ) + lR ((x2n M)(s) ) = f1 (s) + f2 (s) for all sufficiently large s ∈ Z. Let us consider the exact sequence of graded R{x1 , . . . , xn−1 , xn+1 , . . . , x2n }modules 0 → x2n M → Ker θn → Ker θn /x2n M → 0 whose last term is a graded R{x1 , . . . , xn−1 , xn+1 , . . . , x2n−1 }-module. Applying the induction hypothesis to this term we obtain that there exists a numerical

 n−2  t+i i+1 (d0 , . . . , dn−2 ∈ Z) such that f3 (s) = 2 di polynomial f3 (t) = i i=0 ! " (s) lR (Ker θn /x2n M ) for all sufficiently large s ∈ Z. ! " (s) Since f2 (s) = lR ((x2n M)(s) ) = lR ((Ker θn )(s) ) − lR (Ker θn /x2n M) for any s ∈ Z, f2 (t) = f1 (t) − f3 (t). Setting f (t) = f1 (t) + f2 (t) we oball sufficiently large s ∈ Z and f (t) = tain that f (s) =  lR ((x2n M)(s) ) for  n−2  

n−1 n−1    t+i t+i i  t+i i+1 i+1 − = 2 bi 2 di 2 bi 2f1 (t) − f3 (t) = 2 i i i i=0 i=0 i=0 where bn−1 = bn−1 , and bi = bi − di for i = 0, . . . , n − 2. Thus, the polynomial f (t) satisfies conditions (a) and (b). This completes the proof of the theorem.  Definition 3.5.3 Let R be an Artinian inversive difference ring with a basic set σ = {α1 , . . . , αn } and let (Mr )r∈Z be an excellent filtration of a σ ∗ -R-module M . Then the polynomial χ(t) whose existence is established by Theorem 3.5.2 is called the σ ∗ -dimension or characteristic polynomial of the module M associated with the excellent filtration (Mr )r∈Z . Example 3.5.4 Let R be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let E be the ring of σ ∗ -operators over R. Then E can be treated as a σ ∗ -R-module equipped with the excellent filtration (Er )r∈Z , the standard filtration of E. If χE (t) is the corresponding dimension polynomial n then χE (r) = lR (Er ) = dimR (Er ) = Card {γ = α1k1 . . . αnkn ∈ Γσ |ord γ = i=1 |ki | ≤ r} for all sufficiently large r ∈ Z. Proposition 1.4.2 gives three expressions for the last number that imply the following three expressions for χE (t).   

  

 n n  n  n t n t+i n t+i = = . 2i (−1)n−i 2i χE (t) = i n i i i i i=0 i=0 i=0 (3.5.4) Let R be an inversive difference ring with a basic set σ, E the ring of σ ∗ operators over R, and Fm a free left E-module of rank m (m ∈ N) with free generators f1 , . . . , fm . Then for every l ∈ Z, one can consider m an excellent fill l )r )r∈Z of the module Fm such that (Fm )r = i=1 Er−l fi for every tration ((Fm l . A finite r ∈ Z. We obtain a filtered σ ∗ -R-module that will be denoted by Fm

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

199

direct sum of such filtered σ ∗ -R-modules will be called a free filtered σ ∗ -Rmodule. Example 3.5.4 shows that the dimension polynomial χ(t) of the filtered l can be found using one of the following formulas: σ ∗ -R-module Fm  

 n n    n t−l n t+i−l =m 2i i n i i i=0 i=0  

n  n t+i−l . (3.5.5) =m (−1)n−i 2i i i i=0

χ(t) = mχE (t − l) = m

Let R be an inversive difference ring with a basic set σ = {α1 , . . . , αn }, E the ring of σ ∗ -operators over R, M a filtered σ ∗ -R-module with a filtration (Mr )r∈Z , and R[x] the ring indeterminate x over R. Let E  denote the of polynomials in one  r subring Er Rx of the ring E R R[x] and M  denote the left E  -module r∈N   Rr  ring r∈N Mr R Rx . (The ring structure on E R R[x] is induced  by the ∼ E[x]). structure of E[x] via the natural isomorphism of E-modules E R R[x] = The proof of the following lemma is similar to the proof of Lemma 3.2.6. Lemma 3.5.5 With the above notation, let all components of the filtration (Mr )r∈Z be finitely generated R-modules. Then the following conditions are equivalent: (i) The filtration (Mr )r∈Z is excellent; (ii) M  is a finitely generated E  -module.  Lemma 3.5.6 Let R be a Noetherian inversive difference ring with a basic set σ = {α1 , . . . , αn }. Then the ring E  is left Noetherian. PROOF. Let Eˆ be the ring of σ ˆ -operators over R when R is treated as a difference ring with the basic set σ ˆ = {α1 , . . . , αn , αn+1 , . . . , α2n } whose elements αn+1 , . . . , α2n act on the ring R as follows: αn+i (a) = αi−1 (a) for any a ∈ R (i = 1, . . . , n). Clearly, if J is the two-sided ideal of the ring Eˆ generated by the ˆ set {αi αn+i − 1|1 ≤ i ≤ n}, then E is isomorphic to the ring E/J. ∗ It is easy to see that if α is an element of the set σ , then there exists a unique automorphism β of the ring R[x] such that β(x) = x and β(a) = α(a) for any a ∈ R. Let β1 , . . . , β2n be the automorphisms of R[x] obtained as such extensions of α1 , . . . , αn , α1−1 , . . . , αn−1 , respectively. Furthermore, let S denote the ring of skew polynomials R[x][z1 , . . . , z2n ; d1 , . . . , d2n ; β1 , . . . , β2n ] constructed in the proof of Lemma 3.2.7 (di (f ) = xβi (f ) − xf for any f ∈ R[x], i = 1, . . . , 2n). Then the arguments of the proof of Lemma 3.2.7 and the above remark show that E  is isomorphic to the factor ring of S by the two-sided ideal generated by all elements of the form (zi +x)(zn+i +x)−x2 = zi zn+i +xzi +xzn+i (i = 1, . . . , n). By Theorem 1.7.20, the ring of skew polynomials S is left Noetherian, whence the ring E  is also Noetherian.

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Let R be an inversive difference ring with a basic set σ and let M and N be filtered σ ∗ -R-modules with filtrations (Mr )r∈Z and (Nr )r∈Z , respectively. Then a σ-homomorphism f : M → N is said to be a σ-homomorphism of filtered σ ∗ -R-modules if f (Mr ) ⊆ Nr for any r ∈ Z. The proof of the following result is similar to the proof of Theorem 3.2.8. Theorem 3.5.7 Let R be a Noetherian inversive difference ring with a basic set σ and let ρ : N −→ M be an injective homomorphism of filtered σ ∗ -R-modules. Furthermore, suppose that the filtration of the module M is excellent. Then the filtration of N is also excellent.  Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, E the ring of σ ∗ -operators over K, and M a finitely generated σ ∗ -K-module with generators z1 , . . . , z m . Then one can easily see that the ascending chain of m vector K-spaces Mr = i=1 Er zi (r ∈ Z) is an excellent filtration of the module M . This filtration is called a standard filtration of M associated with the system of generators {z1 , . . . , zm }. Let χ(t) be the corresponding characteristic polynomial of M and let d = deg χ(t). By Theorem 3.5.2, d ≤ n and the polyno

d  t+i i where 2 ai mial χ(t) has a unique representation in the form χ(t) = i i=0 a0 , . . . , ad ∈ Z and ad = 0. Let (Mr )r∈Z be another excellent filtration of the σ ∗ -K-module M . Then  for any r ∈ N. there exists some non-negative integer p such that Er Mp = Mr+p

  Since r∈Z Mr = r∈Z Mr = M , there exists some q ∈ N such that Mp ⊆ Mp+q    and Mp ⊆ Mp+q , hence Mr ⊆ Mr+q and Mr ⊆ Mr+q for all r ∈ Z, r ≥ p. Thus, if χ1 (t) is the characteristic polynomials of the σ ∗ -K-module M associated with the excellent filtration (Mr )r∈Z , then χ(r) ≤ χ1 (r + q) and χ1 (r) ≤ χ(r + q) for all sufficiently large r ∈ Z. It follows that deg χ1 (t) = deg χ( t) = d 

d  t+i i where b0 , . . . , bd ∈ Z and 2 bi and χ1 (t) can be written as χ1 (t) = i i=0 ∆d χ(t) ∆d χ1 (t) = . d 2 2d n n Furthermore, we have ∆ χ(t) = ∆ χ1 (t) (if d < n, then both sides of the last equality are equal to zero). We have arrived at the following result. bd = ad . Using the finite differences we can write ad = bd =

Theorem 3.5.8 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, let M be a σ ∗ -K-module, and let χ(t) be a characteristic poly∆n χ(t) , nomial associated with an excellent filtration of M . Then the integers 2n d ∆ χ(t) do not depend on the choice of the excellent filtration d = deg χ(t), and 2d of M .  Definition 3.5.9 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, M a finitely generated σ ∗ -K-module, and χ(t) a characteristic polynomial associated with an excellent filtration of M . Then the

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

201

∆d χ(t) ∆n χ(t) , d = deg χ(t), and are called the inversive difference 2n 2d (or σ ∗ -) dimension, inversive difference (or σ ∗ -) type, and typical inversive difference (or typical σ ∗ -) dimension of the module M , respectively. These characteristics of the σ ∗ -K-module M are denoted by iδ(M ) (or σ ∗ -dimK M ), it(M ) (or σ ∗ -typeK M ) and tiδ(M ) (or σ ∗ -tdimK M ), respectively.

numbers

Definition 3.5.10 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, Γ the free commutative group generated by σ, E the ring of σ ∗ operators over K, and M a σ ∗ -K-module. Elements z1 , . . . , zm ∈ M are said to be σ ∗ -linearly independent over K if the set {γzi |1 ≤ i ≤ m, γ ∈ Γ} is linearly independent over the field K. The following two theorems can be proved precisely in the same way as the similar statements on difference vector spaces (see Theorems 3.2.11 and 3.2.12). Theorem 3.5.11 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let i

j

→M − → P −→ 0 0 −→ N − be an exact sequence of finitely generated σ ∗ -K-modules. Then iδ(N ) + iδ(P ) = iδ(M ).  Theorem 3.5.12 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, E the ring of σ ∗ -operators over K, and M a finitely generated σ ∗ -K-module. Then iδ(M ) is equal to the maximal number of elements of M  σ ∗ -linearly independent over K. Exercise 3.5.13 Let K be a σ ∗ -field with a basic set σ = {α1 , α2 }, E the ring of σ ∗ -operators over K, and M a finitely generated σ ∗ -K-module with two generators z1 and z2 connecting by the defining relation α1 z1 +α2 z2 = 0. (Thus, one can treat M as a factor module of a free left E-module F2 with two free generators f1 and f2 by its E-submodule E(α1 f1 + α2 f2 ). ) Let (Mr )r∈Z be the standard filtration of the σ ∗ -K-module M associated with the system of generators {z1 , z2 }. Find the corresponding σ ∗ -dimension polynomial χ(t) and the invariants iδ(M ), it(M ), and tiδ(M ). Transformations of the basic set Let R be an inversive difference ring with a basic set σ = {α1 , . . . , αn } and let Γ be the free commutative group generated by the set σ. It is easy to see that if σ1 = {τ1 , . . . , τn } is another system of free generators of the group Γ, then there exists a matrix K = (kij )1≤i,j≤n ∈ GL(n, Z) such that αi = τ1ki1 . . . τnkin for i = 1, . . . , n. Since σ1 is a set of pairwise commuting automorphisms of R the ring R can be treated as an inversive difference ring with the basic set σ1 . Clearly, the ring E of σ ∗ -operators over R is the same as the ring of σ1∗ -operators over R. In accordance with the notation of section 2.4, we define the orders of an element γ = α1i1 . . . αnin = τ1j1 . . . τnjn ∈ Γ with respect to the sets σ and σ1

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202

n n as numbers ordσ γ = ν=1 |iν | and ordσ1 γ = ν=1 |jν |, respectively. If u =  γ∈Γ aγ γ ∈ E, then the orders of u with respect to σ and σ1 are defined as usual: ordσ u = max{ordσ γ|aγ = 0} and ordσ1 u = max{ordσ1 γ|aγ = 0}. In the rest of this section, an inversive difference ring R with a basic set σ = {α1 , . . . , αn } will be also denoted by (R, σ). If we treat R as an inversive difference ring with another basic set σ1 , it will be denoted by (R, σ1 ). Furthermore, the ring of σ ∗ -operators over an inversive difference ring (R, σ) will be denoted by Eσ or by Rα1 , . . . , αn , and the free commutative group generated by the set σ will be denoted by Γσ . If K is a σ ∗ -field and M a σ ∗ -K-module, then the inversive difference dimension, the inversive difference type, and the typical inversive difference dimension of M will be denoted by iδσ (M ), itσ (M ), and tiδσ (M ), respectively. Definition 3.5.14 Let (R, σ) and (R, σ1 ) be inversive difference rings with the basic sets σ = {α1 , . . . , αn } and σ1 = {τ1 , . . . , τn }, respectively. The sets σ and σ1 are said to be equivalent if there exists a matrix K = (kij )1≤i,j≤n ∈ GL(n, Z) such that αi = τ1ki1 . . . τnkin (1 ≤ i ≤ n). In this case we write σ ∼ σ1 and say that the transformation of the set σ into the set σ1 is an admissible transformation of σ. The matrix K is called the matrix of this transformation. As we have noticed, if (R, σ) and (R, σ1 ) are inversive difference rings such that σ ∼ σ1 , then Eσ = Eσ1 . In this case, Theorem 3.5.12 shows that iδσ (M ) = iδσ1 (M ) for any finitely generated σ ∗ -(or, equivalently, σ1∗ -)R-module M . Theorem 3.5.15 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let M be a finitely generated σ ∗ -K-module with iδσ (M ) = 0. Then there exists a set σ1 = {τ1 , . . . , τn } of pairwise commuting automorphisms of K with the following properties. (i) The sets σ and σ1 are equivalent. (ii) Let σ2 = {τ1 , . . . , τn−1 }. Then M is a finitely generated σ2∗ -K-module. PROOF. Let E = Eσ be the ring of σ ∗ -operators over K. Let us, first, consider the case when M is a cyclic left E-module with a generator z: M = Ez. Since iδσ (M ) = 0 the element z is σ ∗ -linearly independent over K (see Theorem 3.5.12), that is, there exists a non-zero σ ∗ -operator U ∈ E such that U z = 0. Clearly, one can assume that U ∈ / K (otherwise, M = Ez = E(U −1 U z) = 0 and our statement becomes trivial). m  ai γi , where 0 = ai ∈ K and γi = α1t1i . . . αntni (1 ≤ i ≤ m) Let U = i=1

are distinct elements of the group Γσ (tki ∈ Z for k = 1, . . . , n, i = 1, . . . , m). Without loss of generality, we can assume that all exponents tki (1 ≤ k ≤ n, 1 ≤ i ≤ m) are nonnegative. (If the σ ∗ -operator U does not satisfy this condition, then for every k = 1, . . . , n one can choose the minimal number jk ∈ N such that jk + tki ≥ 0 for all i = 1, . . . , m and consider the σ ∗ -operator U1 = α1j1 . . . αnjn U = m  α1j1 . . . αnjn (ai α1j1 +t1i . . . αnjn +tni ) instead of U .) Furthermore, in what follows i=1

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

203

we assume that the elements γ1 , . . . , γm do not have a common factor in Tσ different from 1. (Indeed, if there exists 1 = γ ∈ Tσ such that γi = γγi for i = 1, . . . , m, then (γ −1 U )z = 0, so one can replace U by the σ ∗ -operator m   γ −1 (ai )γi such that U  z = 0 and γ1 , . . . , γm lie in Tσ and do U  = γ −1 U = i=1

not have a common factor different from 1.) Finally, without loss of generality, we assume that t1i > 0 for some i. Indeed, since all exponents tki are non-negative and U ∈ / K, there exists a strictly positive exponent tki . If k = 1, we can perform an admissible transformation of the basic set σ into the set σ  = {β1 , . . . , βn } such that β1 = αk , βk = α1 and βj = αj if 1 < j ≤ n, j = k. Clearly, σ  ∼ σ and m  ∗ the σ -operator U can be written as U = bi β1s1i . . . βnsni where 0 = bi ∈ K, i=1

sli ∈ N, s1i = tki > 0 (1 ≤ l ≤ n, 1 ≤ i ≤ m) and (s1i , . . . , sni ) = (s1j , . . . , snj ) if i = j. Summarizing our assumptions, we can say that U z = 0 for some σ ∗ -operator m  ai α1t1i . . . αntni ∈ E \ K such that 0 = ai ∈ K, tki ∈ N (1 ≤ i ≤ m, 1 ≤ U = i=1

k ≤ n), t1i > 0 for some i, elements γ1 = α1t11 . . . αntn1 , . . . , γm = α1t1m . . . αntnm do not have a non-trivial common factor in Tσ , and (t1i , . . . , tni ) = (t1j , . . . , tnj ) if i = j. Let Λ denote the set of all elements of the form aα1k1 . . . αnkn with non-zero coefficients a ∈ K (k1 , . . . , kn ∈ Z). Then one can consider a partial ordering  of the set Λ such that aα1k1 . . . αnkn  bα1l1 . . . αnln if and only if (k1 , . . . , kn ) is greater than (l1 , . . . , ln ) with respect to the lexicographic order on Zn . It is easy to see that if Σ is a finite subset of Λ such that λ1 = aλ2 for any λ1 , λ2 ∈ Σ, 0 = a ∈ K, then Σ is well-ordered with respect to . Therefore, we can write the σ ∗ -operator U as U = b1 α1k11 . . . αnk1n + · · · + bm α1km1 . . . αnkmn ,

(3.5.6)

where 0 = bi ∈ K, kij ∈ N (1 ≤ i ≤ m, 1 ≤ j ≤ n) and b1 α1k11 . . . αnk1n  · · ·  bm α1km1 . . . αnkmn . Let dj be the nmaximal exponent of the element αj in the representation (3.5.6), let d(U ) = j=1 dj and let p be a positive integer such that p > 2d(U ). Let us consider the admissible transformation of the basic set σ into another basic set σ1 = {τ1 , . . . , τn } such that α1 α2

2

τ1 τ2p τ3p . . . τnp

=

τ2 τ3p

... αn

n−1

=

=

,

n−2 . . . τnp ,

(3.5.7) τn .

CHAPTER 3. DIFFERENCE MODULES

204

(The transformation is admissible, since the determinant of its matrix is equal to 1.) Expressing our σ ∗ -operator U in terms of τ1 , . . . , τn , we obtain that U = b1 τ1l11 . . . τnl1n + · · · + bm τ1lm1 . . . τnlmn

(3.5.8)

where = = ...

li1 li2

ki1 , ki1 p + ki2 , (3.5.9) n−1

lin

=

ki1 p

(i

=

1, . . . , m).

+ ki2 p

n−2

+ · · · + kin

Since ki1 > 0 for some i (1 ≤ i ≤ m), k11 > 0 whence l1n > 0. Let us show that l1n > l2n ≥ l3n ≥ · · · ≥ lmn . Indeed, since b1 α1k11 . . . αnk1n  b2 α1k21 . . . αnk2n , there exists an index j, 1 ≤ j ≤ n such that k1ν = k2ν for all ν < j and k1j > k2j . Therefore, l1n − l2n =

n 

n 

(k1ν − k2ν )pn−ν > pn−j +

ν=1

(k1ν − k2ν )pn−ν

ν=j+1

≥ pn−j − d(U )

n 

pn−ν > pn−j −

ν=j+1

p pn−j − 1 · >0 2 p−1

whence l1n > l2n . Similarly, for any i = 2, . . . , m−1, one can obtain the inequality lin ≥ li+1 (the equality holds if lin = li+1,n = 0, that is kij = ki+1,j = 0 for all j = 1, . . . , n). " ! Writing the relationship U z = b1 τ1l11 . . . τnl1n + · · · + bm τ1lm1 . . . τnlmn z = 0 in the form b1 τ1l11 . . . τnl1n z = −b2 τ1l21 . . . τnl2n z − · · · − bm τ1lm1 . . . τnlmn z , we obtain that  −l1,n−1 −1 −l11 −l1,n−1 l2,n−1 −l1,n−1 τnl1n z = −τ1−l11 . . . τn−1 (b1 )τ1 . . . τn−1 (b2 )τ1l21 −l11 . . . τn−1  −l1,n−1 −1 −l11 −l1,n−1 (b1 )τ1 . . . τn−1 (bm )τ1lm1 −l11 × τnl2n z + . . . + −τ1−l11 . . . τn−1 l

m,n−1 × . . . τn−1

−l1,n−1

τnlmn z.

(3.5.10)

The last equality shows that the element τnl1n z can be written as a linear combination of the elements z, τn z, . . . , τnl1n −1 z with coefficients from

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

205

Kτ1 , . . . , τn−1 . Multiplying both sides of equality (3.5.10) by τn from the left, we obtain that  l2,n−1 −l1,n−1 τnl2n +1 z + . . . τnl1n +1 z = −τn (b2 )τ1l21 −l11 . . . τn−1  lm,n−1 −l1,n−1 τnlmn +1 z  + −τn (bm )τ1lm1 −l11 . . . τn−1 −l

−l

−l11 1,n−1 1,n−1 where bi = τ1−l11 . . . τn−1 (b−1 . . . τn−1 (bi ) (1 ≤ i ≤ m). 1 )τ1 l1n +1 z can be also written as a linear combination of the Thus, the element τn elements z, τn z, . . . , τnl1n −1 z with coefficients from Kτ1 , . . . , τn−1 . Let

τnl1n +1 z = Vo z + V1 (τn z) + · · · + Vl1n −1 (τnl1n −1 z) ,

(3.5.11)

where V0 , . . . , Vl1n −1 ∈ Kτ1 , . . . , τn−1 . Multiplying both sides of (3.5.11) by τn from the left we obtain that the element τnl1n +2 z can be expressed as a linear combination of the elements z, τn z, . . . , τnl1n −1 z with coefficients from Kτ1 , . . . , τn−1 . Continuing this process we obtain that every element τns z (s ≥ l1n ) can be expressed as such a linear combination. Now, let us consider the σ ∗ -operator U written in the form (3.5.6). As above, for any j = 1, . . . , n, let dj = max{kij |1 ≤ i ≤ m} and let V = α1−d1 . . . αn−dn U =

m 

bi α1ki1 −d1 . . . αnkin −dn .

(3.5.12)

i=1

Then V z = 0 and all exponents of the elements α1 , . . . , αn in the last representation of the σ ∗ -operator V are non-positive. One can also note that the element α1 really appears in the representation (3.5.12) (otherwise ki1 = d1 = k11 > 0 for i = 1, . . . , m, so the monomials bi α1ki1 . . . αnkin would have the common factor α1k11 that contradicts our assumption). Let us write the σ ∗ -operator V as V = c1 α1r11 . . . αnr1n + · · · + cm α1rm1 . . . αnrmn

(3.5.13)

where 0 = ci ∈ K, rij ≤ 0(1 ≤ i ≤ m, 1 ≤ j ≤ n), and cm α1rm1 . . . αnrmn  · · ·  c1 α1r11 . . . αnr1n . It is easy to see that the m-tuple (c1 , . . . , cm ) can be obtained by a permutation if for any j = of the coordinates of the m-tuple (b1 , . . . , bm ). Furthermore, n 1, . . . , n, we set dj = max{|rij ||1 ≤ i ≤ m} and d(V ) = j=1 dj , then d(V ) = d(U ). Let us consider transformation (3.5.7) of the basic set σ (where the integer p satisfies the condition p > 2d(U ) = 2d(V )). Writing the σ ∗ -operator V in terms of τ1 , . . . , τn we arrive at the expression V = c1 τ1s11 . . . τns1n + · · · + cm τ1sm1 . . . τnsmn

(3.5.14)

CHAPTER 3. DIFFERENCE MODULES

206

where sin = rin , si2 = ri1 p + ri2 , . . . sin = ri1 pn−1 + ri2 pn−2 + · · · + rin . Furthermore, the same arguments that were used in the proof of the fact that l1n > l2n ≥ . . . lmn for the σ ∗ -operator U written in the form (3.5.8) show that s1n < s2n ≤ · · · ≤ smn . Writing the equality V z = 0 as c1 τ1s11 . . . τns1n z = −c2 τ1s21 . . . τns2n z + · · · − cm τ1sm1 . . . τnsmn z we obtain that −s

−s

−s11 2,n−1 τns1n z = [−τ1−s11 . . . τn−11,n−1 (c−1 . . . τn−11,n−1 (c2 )τ1s21 −s11 . . . τn−1 1 )τ1 s

−s1,n−1

]

−s −s −s11 × τns2n z + . . . + [−τ1−s11 . . . +τn−11,n−1(c−1 . . . τn−11,n−1(cm )τ1sm1 −s11 1 )τ1 sm,n−1 −s1,n−1 smn . . . τn−1 ]τn z (3.5.15)

where s1n < s2n ≤ · · · ≤ smn ≤ 0. Repeatedly multiplying both sides of (3.5.15) by τn−1 and applying the same equality (3.5.15), we obtain that for any r ∈ Z, r ≤ s1n , the element τnr z can be written as a linear combination of the elements z, τn−1 z, . . . , τns1n +1 z with coefficients in Kτ1 , . . . , τn−1 . Thus, M = Ez =

∞  i=−∞

Kτ1 , . . . , τn−1 τni z

=

l1n −1 

Kτ1 , . . . , τn−1 τni z .

i=s1n +1

It follows that the finite set {τnk z|s1n + 1 ≤ k ≤ lin − 1 } generates M as an inversive difference module with the basic set σ2 = {τ1 , . . . , τn−1 }. Since the set σ1 is obtained by the admissible transformation (3.5.7) of the set σ, σ1 ∼ σ. This completes the proof of the theorem fo r the case of cyclic E-modules. Now, let M be a σ ∗ -K-module generated by m elements z1 , . . . , zm (that m  Ezi ) where m > 1. Then for every i = 1, . . . , m, one can apply is M = i=1

Theorem 3.5.11 to the canonical exact sequence of σ ∗ -K-modules 0 → Ezi → M → M/Ezi → 0 and obtain that 0 ≤ iδσ (Ezi ) = iδσ (M ) − iδσ (M/Ezi ) = −iδσ (M/Ezi ) ≤ 0, so that iδσ (Ezi ) = 0 for i = 1, . . . , m. Now, the first part of the proof shows that for every i = 1, . . . , m, there exist a σ ∗ -operator Ui = ni  q aij α1ij1 . . . αnqijn ∈ E such that all qijk (1 ≤ i ≤ m, 1 ≤ j ≤ ni , 1 ≤ k ≤ n) j=1 qin

1

qin

are non-negative, the monomials α1qi11 . . . αnqi1n , . . . , α1 i . . . αn i do not have non-trivial common factors in Tσ , and U1 z1 = · · · = Um zm = 0. Let us choose an integer p ∈ Z such that p > 2 max{d(Ui )|1 ≤ i ≤ m} (as above, d(Ui ) =  m k=1 max{qijk |1 ≤ j ≤ ni } for i = 1, . . . , m) and consider the corresponding admissible transformation (3.5.7) of the basic set σ into another basic set of automorphisms σ1 = {τ1 , . . . , τn }. By the above observations, if one sets σ2 = {τ1 , . . . , τn−1 }, then every σ ∗ -K-module Ezi (1 ≤ i ≤ m) is finitely generated as a σ2∗ -K-module. Thus, the σ ∗ -K-module M is a finitely generated σ2∗ -K-module as well. This completes the proof of the theorem.  n

The following result can be considered as a generalization of Theorem 3.5.15.

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

207

Theorem 3.5.16 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, M a finitely generated σ ∗ -K-module, and d = itσ (M ). Then there exists a set σ  = {β1 , . . . , βn } of pairwise commuting automorphisms of K with the following properties. (i) The sets σ and σ  are equivalent (so that σ  can be obtained from σ by an admissible transformation). ∗ (ii) Let σ  = {β1 , . . . , βd }. Then M is a finitely generated σ  -K-module and iδσ

(M ) > 0. PROOF. Suppose that a set σ0 = {τ1 , . . . , τn } of automorphisms of the field K is obtained from σ by an admissible transformation such that αi = . . . αnlin (1 ≤ i ≤ . . . , n; j = τ1ki1 . . . τnkin and τi = α1li1  ij ∈ Z for i = 1, nn and kij , l n n n 1, . . . , n). Let q = max{ j=1 |k1j |, . . . , j=1 |knj |, j=1 |l1j |, . . . , j=1 |lnj |} ∗ ∗ and let E denote the ring of σ - (and σ0 -) operators over K (so that E = Eσ = E σ0 ). Furthermore, let (Er )r∈Z and (Er0 )r∈Z be the standard filtrations of the ring E associated with the basic sets σ and σ0 , respectively. (For any r ∈ N, Er = {U ∈ E|ordσ U ≤ r} and Er0 = {U ∈ E|ordσ0 U ≤ r} where ordσ U and ordσ0 U are the orders of an element U ∈ E when U is treated as a σ ∗ -operator and a σ0∗ -operator, respectively. If r < 0, then Er = Er0 = 0.) In what follows, E denotes the ring of σ ∗ -operators over K equipped with the filtration (Er )r∈Z , while the same ring equipped with the filtration (Er0 )r∈Z will be denoted by E 0 . Let z1 , . . . , zm be any finite system of generators of a σ ∗ -K-module M , so m  Ezi . Let χσ (t) and χσ0 (t) be the σ ∗ -dimension polynomials of M that M = i=1

associated with the excellent filtrations (Er )r∈Z and (Er0 )r∈Z , respectively. Then deg χσ (t) = iδσ (M ) = d and deg χσ0 (t) = iδσ0 (M ). 0 It is easy to see that Er ⊆ Erq ⊆ Erq2 for any r ∈ N whence dimK

m 

 Er zi

 ≤ dimK

i=1

m 

 0 Erq zi

≤ dimK

i=1

m 

 Erq2 zi

.

i=1

Therefore, χσ (r) ≤ χσ0 (rq) ≤ χσ (rq 2 ) for all sufficiently large r ∈ Z hence itσ0 (M ) = itσ (M ) = d. Suppose that iδσ (M ) = 0 (otherwise, the statement of our theorem becomes obvious). By Theorem 3.5.15, there exists a set σ1 = {θ1 , . . . , θn } of pairwise commuting automorphisms of the field K such that σ1 ∼ σ and M is a finitely generated σ2∗ -K-module where σ2 = {θ1 , . . . , θn−1 }. Therefore, p m   Eσ2 (θnj zi ) for some positive integer p, so that there exist elements M= i=1 j=−p

wijl , vijl ∈ Eσ2 (1 ≤ i ≤ m, −p ≤ j ≤ p, 1 ≤ l ≤ m) such that θnp+1 zl =

p m   i=1 j=−p

wijl (θnj zi ),

θn−p−1 zl =

p m   i=1 j=−p

vijl (θnj zi ) .

(3.5.16)

CHAPTER 3. DIFFERENCE MODULES

208

for l = 1, . . . , m. Let s = max{ordσ2 wijl , ordσ2 vijl |1 ≤ i ≤ m, −p ≤ j ≤ p, 1 ≤ l ≤ m} and let F denote the ring of σ2∗ -operators over the σ2∗ -field K. Furthermore, let (Fr )r∈Z denote the standard filtration of the ring F (that is Fr = 0 for any r < 0 and if r ∈ N, then Fr is a vector K-space generated by w ≤ r). the set of all σ2∗ -operators 2 ⎛ w such that ordσ⎞ p m   Fr (θnj zi )⎠ is an excellent filtration of the It is easy to see that ⎝ i=1 j=−p

r∈Z

σ2∗ -K-module M and p m   i=1 j=−p

Fr (θnj zi ) ⊆

m 

0 Er+p zi ⊆

i=1

p m  

F(r+p)s (θnj zi ) .

i=1 j=−p

0 (The first inclusion follows from the fact that Fr θnj ⊆ Er+p for j = −p, −p + 1, . . . , p, and the second inclusion is a direct consequence of equalities (3.5.16)). Therefore, (3.5.17) χσ2 (r) ≤ χσ1 (r + p) ≤ χσ2 ((r + p)s) ∗ for all sufficiently large r ∈ Z (χ⎛ σ2 (t) denotes the σ⎞ 2 -dimension polynomial of p m   M associated with the filtration ⎝ Fr (θnj zi )⎠ . i=1 j=−p

r∈Z

It is easy to see that inequalities (3.5.17) and the equivalence σ1 ∼ σ imply the equalities itσ2 (M ) = itσ1 (M ) = d. Furthermore, if iδσ2 (M ) > 0 (that is, d = n − 1), then the sets σ  = σ1 and σ  = σ2 satisfy conditions (i) and (ii) of the theorem. If iδσ2 (M ) = 0, then we can repeat the above reasonings and arrive at the set σ3 = {λ1 , . . . , λn−1 } of pairwise commuting automorphisms of K such that σ3 ∼ σ2 and M is a finitely generated σ4∗ -module, where σ4 = {λ1 , . . . , λn−2 }. Furthermore, itσ3 (M ) = itσ2 (M ) = d. If iδσ4 (M ) > 0 (that is, d = n − 2), then we set σ  = {β1 , . . . , βn } where βi = λi for i = 1, . . . , n − 1 and βn = θn . Clearly, this set and the set σ  = σ4 satisfy conditions (i) and (ii) of our theorem. In this case, the admissible transformation of σ into σ  is the composition of the admissible transformations of σ into σ1 and σ1 into σ  (the last transformation is obtained by adjoining the relationship βn = θn to the admissible transformation of σ2 into σ3 ). If iδσ4 (M ) = 0 (that is, d < n−2), we can repeat the the above procedure and so on. After a finite number of steps we obtain a set σ  = {β1 , . . . , βd , . . . , βn } of pairwise commuting automorphisms of the field K such that σ  ∼ σ, M is ∗ a finitely generated σ  -K-module, where σ  = {β1 , . . . , βd }, and iδσ

(M ) > 0  (that is itσ

(M ) = d). This completes the proof of the theorem. Exercise 3.5.17 Let K be an inversive difference field with a basic set σ = {α1 , α2 }, let E be the ring of σ ∗ -operators over K, and let M be a finitely generated σ ∗ -K-module with two generators z1 , z2 and two defining relations α1 z1 + α2 z2 = 0 and α2 z1 + α1 z2 = 0. (Thus, one can treat, M as a factor module of a free left E-module F2 with two free generators f1 and f2 by its

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

209

E-submodule E(α1 f1 + α2 f2 ) + E(α2 f1 + α1 f2 ). ) Show that iδσ (M ) = 0 and find an admissible transformation of the set σ into some new set σ1 = {τ1 , τ2 } of commuting automorphisms of K such that M is a finitely generated σ2∗ -Kmodule where σ2 = {τ1 }. We conclude this section with results on multivariable dimension polynomials of inversive difference vector spaces associated with partitions of basic sets of automorphisms. Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, Γ the free commutative group of all power products α1k1 . . . αnkn (k1 , . . . , kn ∈ Z) and E the ring of σ ∗ -operators over K. Let us fix a partition of the set σ into a disjoint union of its subsets: σ = σ 1 ∪ · · · ∪ σp

(3.5.18)

where p ∈ N, and σ1 = {α1 , . . . , αn1 }, σ2 = {αn1 +1 , . . . , αn1 +n2 }, . . . , σp = {αn1 +···+np−1 +1 , . . . , αn } (ni ≥ 1 for i = 1, . . . , p ; n1 + · · · + np = n). If γ = α1k1 . . . αnkn ∈ Γ (k1 , . . . , kn ∈ Z), then for any i = 1, . . . , p, the number n1 +···+ni |kν | (1 ≤ i ≤ p) is called the order of γ with respect ordi γ = ν=n 1 +···+ni−1 +1 to σi (we assume that n0 = 0, so the indices in the sum for ord1 γ change n from 1 ). As before, the order of the element γ is defined as ord γ = to n 1 ν=1 |ki | = p i=1 ordi γ. We shall consider p orders <1 , . . . ,

CHAPTER 3. DIFFERENCE MODULES

210 (ii)



Mr1 ,...,rp = M .

(r1 ,...,rp )∈Zp (0)

(0)

(iii) There exists a p-tuple (r1 , . . . , rp ) ∈ Zp such that Mr1 ,...,rp = 0 if ri < (0) ri for at least one index i (1 ≤ i ≤ p). (iv) Er1 ,...,rp Ms1 ,...,sp ⊆ Mr1 +s1 ,...,rp +sp for any p-tuples (r1 , . . . , rp ), (s1 , . . . , sp ) ∈ Zp . If every vector K-space Mr1 ,...,rp is finite-dimensional and there exists an element (h1 , . . . , hp ) ∈ Zp such that Er1 ,...,rp Mh1 ,...,hp = Mr1 +h1 ,...,rp +hp for any (r1 , . . . , rp ) ∈ Np , the p-dimensional filtration {Mr1 ,...,rp |(r1 , . . . , rp ) ∈ Zp } is called excellent. It is easy to see that if z1 , . . . , zk is a finite system of generators of a vector σ ∗ k  K-space M , then { Er1 ,...,rp zi |(r1 , . . . , rp ) ∈ Zp } is an excellent p-dimensional i=1

filtration of M . Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, Γ the free commutative group generated by σ, and E the ring of σ ∗ -operators over K. Furthermore, we assume that partition (3.5.18) of the set σ is fixed. In what follows, a free E-module is also called a free σ ∗ -K-module or a free vector σ ∗ -K-space. If such a module F has a finite family {f1 , . . . , fm } of free generators, it is called a finitely generated free vector σ ∗ -K-space. In this case the elements of the form γfν (γ ∈ Γ, 1 ≤ ν ≤ m) are called terms while the elements of the group Γ are called monomials. The set of all terms is denoted by Γf ; it is easy to see that this set generates F as a vector space over the field K. By the order of a term γfν with respect to σi (1 ≤ i ≤ p) we mean the order of the monomial γ with respect to σi . We shall consider p orderings of the set Γf that correspond to the orderings of the group Γ introduced above. These orderings are denoted by the same symbols <1 , . . . ,

(n)

of the set Zn where Z1 , . . . , Z2n are all distinct Cartesian products of n factors ¯ − = {a ∈ Z | a ≤ 0} or N (we assume that Z1 = Nn ). each of which is either Z Then the group Γ and the set of terms Γf can be represented as the unions 2  n

Γ=

2  n

Γj

j=1 kpn

and

Γf =

Γj f

j=1 (n)

k11 . . . αpnpp |(k11 , . . . , kpnp ) ∈ Zj } and Γj f = {γfi | γ ∈ where Γj = {γ = α11 Γj , 1 ≤ i ≤ m}. (Such a representation of Γ was also considered in Section 2.4 where we studied autoreduced sets of inversive difference polynomials.)

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

211

Two elements γ, γ  ∈ Γ are said to be similar if they belong to the same set Γj (1 ≤ j ≤ 2n ). In this case we write γ  γ  or γ j γ  . Note that  is not a transitive relation on Γ. Let F be a finitely generated free vector σ ∗ -K-space and let f1 , . . . , fm be free generators of F over the ring of σ ∗ -operators E. An element γ ∈ Γ and a term γ  fi ∈ Γf (γ  ∈ Γ, 1 ≤ i ≤ m) are called similar if γ j γ  for some j = 1, . . . , 2n . It is written as γ  γ  fi or γ j γ  fi . Furthermore, we say that two terms γfi , γ  fk ∈ Γf (1 ≤ i, k ≤ m) are similar and write γfi  γ  fk or γfi j γ  fk , if γ j γ  for some j = 1, . . . , 2n . It is easy to see that if γ  u for some term u ∈ Γf (or γ  γ  for some γ  ∈ Γ), then ordν (γu) = ordν γ + ordν u (respectively, ordν (γγ  u) = ordν γ + ordν γ  ) for ν = 1, . . . , p. Definition 3.5.19 Let γ1 , γ2 ∈ Γ. We say that γ1 is a transform of γ2 and write γ2 | γ1 , if γ1 and γ2 belong to the same set Γj (1 ≤ j ≤ 2n ) and there γ1 exists γ ∈ Γj such that γ1 = γγ2 . (In this case we write γ = .) Furthermore, γ2 we say that a term u = γ1 fi is a transform of a term v = γ2 fk and write v | u, u if i = k and γ1 is a transform of γ2 . (If u = γv, we write γ = .) v In what follows, if two terms u = γ1 fi and v = γ2 fi belong to the same set Γj f (1 ≤ j ≤ 2n ) and γ1 = γγ2 for some γ ∈ Γj (so that v|u), then the u monomial γ will be sometimes written as a fraction . v Since the set Γf is a basis of F over the field K, any nonzero element h ∈ F has a unique representation in the form h = a1 γ1 fi1 + · · · + al γfil

(3.5.20)

where γν ∈ Γ, aν ∈ K, aν = 0 (1 ≤ ν ≤ l), 1 ≤ i1 , . . . , il ≤ m, and γν fiν = γµ fiµ whenever ν = µ (1 ≤ ν, µ ≤ l). Definition 3.5.20 Let h be a nonzero element of the E-module F written in the form (3.5.20). Then for every k ∈ {1, . . . , p}, the greatest with respect to
212

CHAPTER 3. DIFFERENCE MODULES

i = j). Prove that for every k = 1, . . . , p, there exist unique elements γν fiν and (1) γi (1 ≤ ν ≤ l, 1 ≤ i ≤ s) such that uωh = γi γν fiν . Definition 3.5.23 Let f, g ∈ F and let k, i1 , . . . , il be distinct elements in the set {1, . . . , p}. Then the element f is said to be ( j, hi is not (<1 , . . . ,

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

213

Thus, we can assume that all 1-leaders of our infinite (<1 , . . . ,

(j)

Let kij = ordj ugi and lij = ordj ugi (i = 1, 2, . . . , 2 ≤ j ≤ p). Obviously, lij ≥ kij (i = 1, 2, . . . ; j = 2, . . . , p), so that {(li2 − ki2 , . . . , lip − kip )|i = 1, 2, . . . } ⊆ Np−1 . By Lemma 1.5.1, there exists an infinite sequence of indices i1 < i2 < . . . such that (li1 2 − ki1 2 , . . . , li1 p − ki1 p ) ≤P (li2 2 − ki2 2 , . . . , li2 p − ki2 p ) ≤P . . . . (1)

(1)

Let γ be an element of Γq such that ugi2 = γugi1 . Then for any j = 2, . . . , p, (1)

(j)

we have ordj γ +ordj ugi1 = ki2 j −ki1 j +li1 j ≤ ki2 j +li2 j −ki2 j = li2 j = ordj ugi2 , so that gi2 contains a term (j)

(1) γugi1

=

(1) ugi2

such that γ ∈ Γq and

(j) ordj (γugi1 )

≤

ordj ugi2 for j = 2, . . . , p. Thus, gi2 is not (<1 , . . . ,

(<1 , . . . ,

(1)

transform γ  ugi (γ  ∈ Γ, γ  ugi , 1 ≤ i ≤ r) which is greater than wf with (ν) (ν) respect to <1 and satisfies the condition ordν γ  + ordν ugi ≤ ordν uf
for ν = (ν)

(ν)

2, . . . , p. Indeed, the last inequality implies that ordν γ  + ordν ugi ≤ ordν uf (1)

for ν = 2, . . . , p, so that the term γ  ugi cannot appear in f . This term cannot (1) (1) (1) appear in γgj either, since uγgj = γugj = wf <1 γ  ugi .

CHAPTER 3. DIFFERENCE MODULES

214 (1)

−1

Thus, γ  ugi cannot appear in f  = f − cf (γ(lc1 (gj ))) γgj , whence the Σleader of f  is strictly less with respect to the order <1 than the Σ-leader of f . Applying the same procedure to the element f  and continuing in the same way, we obtain an element g ∈ F such that f − g is a linear combination of elements g1 , . . . , gr with coefficients in E and g is (<1 , . . . ,

(µ)

ug or there exists some ν, 2 ≤ ν ≤ p, such that uf (ν) uf

(ν) <ν ug .

(i) uf

(µ)

= ug

for µ = 1, . . . , ν − 1

(i) ug

If = for i = 1, . . . , p, we say that f and g have the and same rank and write rk(f ) = rk(g). In what follows, while considering (<1 , . . . ,

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

215

Definition 3.5.30 Let Σ = {h1 , . . . , hr } and Σ = {h1 , . . . , hs } be two (<1 , . . . ,

s and rk(hi ) = rk(hi ) for i = 1, . . . , s. If r = s and rk(hi ) = rk(hi ) for i = 1, . . . , r, then Σ is said to have the same rank as Σ . In this case we write rk(Σ) = rk(Σ ). Theorem 3.5.31 In every nonempty set of (<1 , . . . ,

0.) It is clear that every  element of Φj−1 is an autoreduced subset in Φ of lowest rank. Since every E-submodule of F contains at least one (<1 , . . . ,

CHAPTER 3. DIFFERENCE MODULES

216

Theorem 3.5.34 Let N be a E-submodule of the free E-module F and let Σ = {g1 , . . . , gr } be a (<1 , . . . ,

shows that g = 0. Thus, f =

r 



λi gi .

i=1

Theorem 3.5.35 Let Σ1 = {g1 , . . . , gr } and Σ2 = {h1 , . . . , hs } be two normal (<1 , . . . ,

(1)

Since Σ is a (<1 , . . . ,

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS (1)

(1)

(1)

217

(1)

a transform of some ugl with l ≥ 2 hence ug2 ≤1 ugl ≤1 uh2 . Because of the (j)

(j)

minimality of rank of Σ one has uh2 = ug2 for j = 1, . . . , p. Continuing in the same way we obtain that s ≤ r and rk(hi ) = rk(gi ) for i = 1, . . . , s. Since Σ is a (<1 , . . . ,

The following result can be proved in the same way as Theorem 3.3.15 (one just needs to use of Theorem 3.5.33 instead of Proposition 3.3.10). We leave the corresponding adjustment of the proof of Theorem 3.3.15 to the reader as an exercise. Theorem 3.5.37 Let K be an inversive difference field with a basic set σ, whose partition (3.5.18) into p disjoint subsets is fixed, and let E be the ring of σ ∗-operators over K. Let M be a finitely generated E-module with a system of generators {h1 , . . . , hm }, F a free E-module with a basis f1 , . . . , fm , and π : F −→ M the natural E-epimorphism of F onto M (π(fi ) = hi for i = 1, . . . , m). Furthermore, let N = Ker π and let Σ = {g1 , . . . , gd } be a (<1 , . . . ,

(1)

and u is not a transform of any ugi (1 ≤ i ≤ d) }, Wr1 ...rp = {u ∈ Γf \ Vr1 ...rp |ordi u ≤ ri for i = 1, . . . , p and whenever u = (1) (1) γug is a transform of some ug (g ∈ G, γ ∈ Γ), there exists i ∈ {2, . . . , p} (i) such that ordi γ + ordi ug > ri },
Ur1 ...rp = Vr1 ...rp Wr1 ...rp . Then for any (r1 , . . . , rp ) ∈ Np , the set π(Ur1 ...rp ) is a basis of the vector  K-space Mr1 ...rp . By Theorem 1.5.14, there is a numerical polynomial φV (t1 , . . . , tp ) in p variables t1 , . . . , tp such that φV (r1 , . . . , rp ) = Card Vr1 ...rp for all sufficiently large (r1 , . . . , rp ) ∈ Zp (we use the notation of the last theorem), The total degree of the polynomial φV does not exceed n and degti φV ≤ ni for i = 1, . . . , p. Furthermore, repeating the arguments of the proof of Theorem 3.3.16, we obtain there is a certain combination

 linear

 φW (t1 , . . . , tp ) of polynomials of the form tp + np + cp t1 + n1 + c1 ... (c1 , . . . , cp ∈ Z) with integer coefficients n1 np such that φW (r1 , . . . , rp ) = Card Wr1 ...rp for all sufficiently large (r1 , . . . , rp ) ∈ Zp . We obtain the following statement which is an analog of the combination of Theorems 3.3.16 and 3.3.21. (As in section 3.3, for any set Σ ⊆ Np , Σ denotes the set of all maximal elements of Σ with respect to one of the lexicographic orders <j1 ,...,jp where {j1 , . . . , jp } is a permutation of {1, . . . , p}.) Theorem 3.5.38 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let E be the ring of σ ∗ -operators over K equipped with the

CHAPTER 3. DIFFERENCE MODULES

218

standard p-dimensional filtration corresponding to partition (3.3.18) of the set σ. Furthermore, let ni = Card σi (i = 1, . . . , p) and let {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp } be an excellent p-dimensional filtration of a vector σ ∗ -K-space M . Then there exists a polynomial ψ(t1 , . . . , tp ) ∈ Q[t1 , . . . , tp ] such that (i) ψ(r1 , . . . , rp ) = dimK Mr1 ...rp for all sufficiently large (r1 , . . . , rp ) ∈ Zp ; (ii) degti ψ ≤ ni (1 ≤ i ≤ p), so that deg ψ ≤ n and the polynomial ψ(t1 , . . . , tp ) can be represented as ψ(t1 , . . . , tp ) =

n1  i1 =0

...

np  ip =0

ai1 ...ip

  t1 + i1 tp + ip ... i1 ip

where ai1 ...ip ∈ Z for all i1 , . . . , ip . (iii) The total degree d of the polynomial ψ, the coefficient an1 ...np , p-tuples (j1 , . . . , jp ) ∈ Σ , the corresponding coefficients aj1 ...jp , and the coefficients of the terms of total degree d do not depend on the choice of the excellent filtration. an1 ...np is equal to σ ∗ -dimK M , that is, to the Furthermore, 2n | an1 ...np and 2n maximal number of elements of M linearly independent over E. Definition 3.5.39 The polynomial ψ(t1 , . . . , tp ), whose existence is established by Theorem 3.5.38, is called a dimension (or (σ1 , . . . , σp )∗ -dimension) polynomial of the σ ∗ -K-vector space M associated with the p-dimensional filtration {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp }. The (σ1 , . . . , σp )∗ -dimension polynomial ψ(t1 , . . . , tp ) of an excellently filtered E-module M can be computed by constructing a (<1 , . . . ,
3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

219

Let M be a finitely generated E-module with a system of generators {h1 , . . . , hm } and let {Mr1 ...rp |(r1 , . . . , rp ) ∈ Zp } be an excellent p-dimensional m  Er1 ...rp hi filtration of M associated with these generators (Mr1 ...rp = i=1

for all (r1 , . . . , rp ) ∈ Zp ). Furthermore, let F be a free E-module with a basis f1 , . . . , fm , π : F −→ M the natural E-epimorphism of F onto M (π(fi ) = hi for i = 1, . . . , m), and {g1 , . . . , gd } a set of generators of the E-module N = Ker π. m  ωjk fk where ωjk ∈ E (1 ≤ j ≤ d, 1 ≤ k ≤ m). Let gj = k=1

Let us denote the automorphisms α1−1 , . . . , αn−1 of the field K by symbols αn+1 , . . . , α2n , respectively, and let us consider K as a difference field with a basic set σ ˆ = {α1 , . . . , α2n }. Let T denote the free commutative semigroup ˆ -operators generated by the elements α1 , . . . , α2n and let D denote the ring of σ over K. Furthermore, let us fix the following partition of the set σ ˆ: σ ˆ=σ ˆ1 ∪ · · · ∪ σ ˆp

(3.5.21)

where σ ˆ1 = {α1 , . . . , αn1 , αn+1 , . . . , αn+n1 }, . . . , σˆp = {αn1 +···+np−1 +1 , . . . , αn , αn+n1 +···+np−1 +1 , . . . , α2n }. In what follows we shall consider D as a filtered ring with the standard p-dimensional filtration {Dr1 ...rp | (r1 . . . rp ) ∈ Zp } corresponding to partition (3.5.21). ˆ be a Let E be a free D-module with m free generators e1 , . . . , em and let N m  ω ˆ jk ek (1 ≤ j ≤ d, 1 ≤ D-submodule of E generated by d+mn elements gˆj = k=1

ˆ jk is a σ ˆ -operator k ≤ m), zˆiν = (αi αn+i − 1)eν (1 ≤ i ≤ n, 1 ≤ ν ≤ m) where ω in D obtained by replacing every αi−1 (1 ≤ i ≤ n) in ωjk with αn+i (1 ≤ j ≤ d, ˆ = E/N ˆ is a D-module generated by the cosets eˆk = ek + N ˆ 1 ≤ k ≤ m). Then M m  ˆ r ...r = Dr1 ...rp eˆk form an excellent (1 ≤ k ≤ m), and the vector K-spaces M 1 p k=1

ˆ (in the sense of Definition 3.3.1). Furthermore, it p-dimensional filtration of M ˆ is easy to see that dimK Mr1 ...rp = dimK Mr1 ...rp for every (r1 . . . rp ) ∈ Zp . Thus, the (σ1 , . . . , σp )∗ -dimension polynomial ψ(t1 , . . . , tp ) associated with the filtration {Mr1 ...rp | (r1 . . . rp ) ∈ Zp } of the E-module M coincides with the (σ1 , . . . , σp )-dimension polynomial φ(t1 , . . . , tp ) associated with the filtraˆ r ...r | (r1 . . . rp ) ∈ Zp } of the D-module M ˆ . It follows that one can tion {M 1 p compute the (σ1 , . . . , σp )∗ -dimension polynomial of the module M using the technique of Gr¨ obner bases with respect to the orderings <1 , . . . ,

CHAPTER 3. DIFFERENCE MODULES

220 Let us fix a partition

σ = σ1 ∪ σ 2 ,

(3.5.22)

where σ1 = {α1 } and σ2 = {α2 }, and compute the (σ1 , σ2 )∗ -dimension polynomial ψ(t1 , t2 ) of the module M associated with the excellent 2-dimensional filtration {Mrs = Ers h1 + Ers h2 | (r, s) ∈ Z2 }. Following the above scheme, we denote the automorphisms α−1 and α−2 of the field K by α3 and α4 , respectively, consider K as a difference field with a basic set σ ˆ = {α1 . . . , α4 }, and set a partition ˆ2 , σ ˆ=σ ˆ1 ∪ σ

(3.5.23)

ˆ2 = {α2 , α4 }. Furthermore, we denote the set of where σ ˆ1 = {α1 , α3 } and σ σ ˆ -operators over K by D, consider a free D-module E with two free generators ˆ of E generated by the elements g1 = α1 e1 − α2 e2 , e1 , e2 and a D-submodule N g2 = α1 α3 e1 − e1 , g3 = α2 α4 e1 − e1 , g4 = α1 α3 e2 − e2 , and g4 = α2 α4 e2 − e2 . Let G0 = {g1 , g2 , g3 , g4 , g5 }. (1) (2) (1) (2) (1) (2) Clearly, ug1 = α1 e1 , ug1 = α2 e2 , ug2 = ug2 = α1 α3 e1 , ug3 = ug3 = (1) (2) (1) (2) α2 α4 e1 , ug4 = ug4 = α1 α3 e2 , ug5 = ug5 = α2 α4 e2 . Applying the Gr¨ obner basis method developed in Section 3.3. (and using the notation of Definition 3.3.11 and Theorem 3.3.13) we find that S2 (g1 , g2 ) = S2 (g1 , g3 ) = S2 (g2 , g4 ) = S2 (g2 , g5 ) = S2 (g3 , g4 ) = S2 (g3 , g5 ) = 0 (the 2-leaders of the elements of each of these pairs have different free generators of E, so their least common multiple is 0), g3

g2

<2

<2

S2 (g2 , g3 ) = α2 α4 g2 − α1 α3 g3 = α1 α3 e1 − α2 α4 e1 −−→ α1 α3 e1 − e1 −−→ 0 G

0 and similarly S2 (g4 , g5 ) −−→ 0. Furthermore,

<2

g2

g1

<2

<2

S2 (g1 , g4 ) = α1 α3 g1 + α2 g4 = α12 α3 e1 − α2 e2 −−→ −α2 e2 + α1 e1 −−→ 0. The only 2-d S-polynomial of elements of G0 which does not <2 -reduce to 0 with respect to G0 is S2 (g1 , g5 ) = α4 g1 + g5 = α1 α4 e1 − e2 . We denote this element by g6 and set G1 = G0 ∪ {g6 }. (2) (1) Obviously, S2 (g1 , g6 ) = S2 (g4 , g6 ) = S2 (g5 , g6 ) = 0 (ug6 = ug6 = α1 α4 e1 while the 2-leaders of g1 , g4 , and g5 contain e2 ). Furthermore, g1

S2 (g3 , g6 ) = α1 g3 − α2 g6 = −α1 e1 + α2 e2 −−→ 0, <2

but S2 (g2 , g6 ) = α4 g2 − α3 g6 = −α4 e1 − α3 e2 does not reduce to 0 with respect to G1 . We set g7 = −S2 (g2 , g6 ) = α4 e1 + α3 e2 and G2 = G1 ∪ {g7 }. (1) (2) Now we have ug7 = α3 e2 and ug7 = α4 e1 whence S2 (g1 , g7 ) = S2 (g4 , g7 ) = S2 (g5 , g7 ) = 0, g4

g4

<2

<2

S2 (g2 , g7 ) = α4 g2 −α1 α3 g7 = −α4 e1 +α1 α32 e2 −−→ −α4 e1 +α3 e2 −−→ 0,

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS g1

g2

<2

<2

221

S2 (g3 , g7 ) = g3 − α2 g7 = α2 α3 e2 − e1 −−→ α1 α3 e1 − e1 −−→ 0, g4

S2 (g6 , g7 ) = g6 − α1 g7 = α1 α3 e2 − e2 −−→ 0. <2

ˆ with respect to <2 . obner basis of N Thus, G2 = {g1 , . . . , g7 } is a Gr¨ Considering the 1-leaders of elements of G, we see that S1 (g1 , g4 ) = S1 (g1 , g5 ) = S1 (g1 , g7 ) = S1 (g2 , g4 ) = S1 (g2 , g5 ) = S1 (g2 , g7 ) = S1 (g3 , g4 ) = S1 (g3 , g5 ) = S1 (g3 , g7 ) = S1 (g4 , g6 ) = S1 (g5 , g6 ) = S1 (g6 , g7 ) = 0 (the 1-leaders of elements of G

each of these pairs include different free generators of E) and S1 (g2 , g3 ) −−−2−→ 0, <1 ,<2

G

G

G

<1 ,<2

<1 ,<2

<1 ,<2

S1 (g2 , g6 ) −−−2−→ 0, S1 (g3 , g6 ) −−−2−→ 0, S1 (g4 , g5 ) −−−2−→ 0 (the 1-leaders of the elements g2 , . . . , g6 are the same as their 2-leaders, so the last four reductions can be obtained as above, when we considered the corresponding reductions with respect to <2 ). Furthermore, g1 g5 S1 (g1 , g3 ) = α2 α4 g1 − α1 g3 = α1 e1 − α22 α4 e2 −−−−→ α2 e2 − α22 α4 e2 −−−−→ 0, g5

<1 ,<2

<1 ,<2

S1 (g1 , g6 ) = α4 g1 − g6 = −α2 α4 e2 + e2 −−−−→ 0, <1 ,<2 g7

S1 (g2 , g6 ) = α4 g2 − α3 g6 = α3 e2 − α4 e1 −−−−→ 0, <1 ,<2 g1

S1 (g3 , g6 ) = α1 g3 − α2 g6 = α2 e2 − α1 e1 −−−−→ 0, <1 ,<2 g6

S1 (g4 , g7 ) = g4 − α1 g7 = α1 α4 e1 − e2 −−−−→ 0, <1 ,<2

S1 (g5 , g7 ) = α3 g5 − α2 α4 g7 =

α2 α42 e1

g3

g7

<1 ,<2

<1 ,<2

− α3 e2 −−−−→ α4 e1 − α3 e2 −−−−→ 0,

S1 (g1 , g2 ) = α3 g1 − g2 = −α2 α32 e2 + e1 . The last element does not (<1 , <2 )-reduces to 0, so we set g8 = α2 α32 e2 − e1 and G = G2 ∪ {g8 }. Now one can easily see that S1 (g1 , g8 ) = S1 (g2 , g8 ) = g1 S1 (g3 , g8 ) = S1 (g6 , g8 ) = 0, S1 (g4 , g8 ) = α2 g4 − α1 g8 = α1 e1 − α2 e2 −−−−→ 0, <1 ,<2

g7

S1 (g5 , g8 ) = α3 g5 − α4 g8 = α4 e1 − α3 e2 −−−−→ 0, and S1 (g7 , g8 ) = α2 g7 − g8 = <1 ,<2

g3

−α2 α4 e1 + e2 −−−−→ 0. <1 ,<2

ˆ with respect to (<1 , <2 ). Thus, G = {g1 , . . . , g8 } is a Gr¨ obner basis of N a b c d If for any term u = τ ei = α1 α2 α3 α4 ei ∈ T (a, b, c, d ∈ N, i = 1, 2) we consider the corresponding 4-tuple (a, b, c, d) of exponents of τ , then the set of such 4-tuples associated with the 1-leaders of elements of G containing e1 is as follows: A1 = {(1, 0, 0, 0), (1, 0, 1, 0), (0, 1, 0, 1), (1, 0, 0, 1)} (1)

(1)

(1)

(1)

(the elements of A1 correspond to ug1 , ug2 , ug3 , and ug6 ). A similar set A2 associated with the 1-leaders of elements of G containing e2 is of the form A2 = {(1, 0, 1, 0), (0, 1, 0, 1), (0, 0, 1, 0), (0, 1, 1, 0)}. Using the notation of the proof of Theorem 3.3.16, we obtain that the numerical polynomial ω(t1 , t2 ), whose values describe the number of elements

CHAPTER 3. DIFFERENCE MODULES

222

 for all sufficiently large r, s ∈ N, is the sum of the dimenof the set Urs sion polynomials ωA1 (t1 , t2 ) and ωA2 (t1 , t2 ) of the sets A1 and A2 , respectively (see Definition 1.5.3). Applying formula (1.5.3) one can easily find that ωA1 (t1 , t2 ) = ωA2 (t1 , t2 ) = 2t1 t2 + t1 + 2t2 + 1. Therefore,

ω(t1 , t2 ) = 4t1 t2 + 2t1 + 4t2 + 2.  With the notation of Theorem 3.3.16, the set Urs is the union of two (1) (2) (1) (1) a b c d sets Urs and Urs such that Urs = {u = α1 α2 α3 α4 e1 ∈ T | u = τ ugi (1) (2) is a transform of some 1-leader ugi containing e1 and ord2 (τ ugi ) > s} = (2) {u = α1a α2b α3c α4d e1 ∈ T | 1 ≤ a ≤ r, c = 0, d = 0, and b = s}, Urs = (1) (1) {u = α1a α2b α3c α4d e2 ∈ T | u = τ ugi is a transform of some 1-leader ugj (2) containing e2 and ord2 (τ ugj ) > s} = {u = α1a α2b α3c α4d e1 ∈ T | 1 ≤ c ≤ r, a = 0, b = 0, and d = s}.   It follows that Card Urs = 2r, so the size of Urs is described by the polynomial φ(t1 , t2 ) = 2t1 . As it is shown in the proof of Theorem 3.3.16, ˆ is ω(t1 , t2 ) + φ(t1 , t2 ). Therefore, the ˆ2 )-polynomial the D-module N the (ˆ σ1 , σ ∗ (σ1 , σ2 ) -dimension polynomial ψ(t1 , t2 ) of the module M associated with the excellent 2-dimensional filtration {Mrs = Ers h1 + Ers h2 | (r, s) ∈ Z2 } is as follows: ψ(t1 , t2 ) = 4t1 t2 + 4t1 + 4t2 + 2.

Furthermore, as it follows from Theorem 3.5.38, σ ∗ -dimK M =

4 = 1. 22

As we have seen, the computation of dimension polynomials associated with finitely generated inversive difference vector spaces is quite a long process. However, one can always reduce the problem to the computation of dimension polynomials of difference modules and use computer algebra systems for the realization of the difference analog of the Buchberger Algorithm. Another possible approach is to develop a Gr¨ obner basis method for inversive difference vector spaces using the concept of reduction implied by in Definition 3.5.23. Exercise 3.5.42 With the notation of Definitions 3.5.19, 3.5.20, and 3.5.23, let us consider p − 1 new symbols z1 , . . . , zp−1 and the free commutative semigroup lp−1 with γ ∈ Γ; l1 , . . . , lp−1 ∈ N. Let Λ of all power products λ = γz1l1 . . . zp−1 Λf = {λfj | λ ∈ Λ, 1 ≤ j ≤ m} = Λ × {f1 , . . . , fm }. Furthermore, for any (i) (1) element f ∈ F , let di (f ) = ordi uf − ordi uf (2 ≤ i ≤ p) and let ρ : F → Λf d (f )

d

(1)

p(f ) uf . be defined by ρ(f ) = z1 2 . . . zp−1 Let N be a E-submodule of F . A finite set G = {g1 , . . . , gt } ⊆ N will be called a Gr¨ obner basis of N with respect to the orders <1 , . . . ,

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS (k)

223

(k)

iff ug |w for some term w in f with a coefficient a, w = γug (γ ∈ Γ), (i ) (i ) h = f − a(γ(lck (g)))−1 γg and ordiν γ + ordiν ug ν ≤ ordiν uf ν (1 ≤ ν ≤ l). If f, h ∈ F and G ⊆ F , then we say that the element f (
reduces to h modulo G and write f −−−−−−−−→ h iff there exist two se
g (1)

quences g (1) , g (2) , . . . g (q) ∈ G and h1 , . . . , hq−1 ∈ F such that f −−−−−−−−− →
g (2)

g (q−1)

g (q)




h1 −−−−−−−−− → . . . −−−−−−−−− → hq−1 −−−−−−−−− → h. Let us define the least common multiple of two terms u = γu1 fi = α1k1 . . . αnkn fi and v = γ  fj = α1l1 . . . αnln in Γf , as follows: lcm(u, v) = 0 if either i = j or i = j and the n-tuples (k1 , . . . , kn ) and (l1 , . . . , ln ) do not belong to the same ortant of Zn . If i = j and the n-tuples of exponents of u and v belong to the same ortant (n)  max{|k1 |,|l1 |}  max{|kn |,|ln |} . . . αnn fi where (1 , . . . , n ) is Zj , then lcm(u, v) = α11 (n)

an n-tuple with entries 1 and −1 that belongs to Zj . Now, for any nonzero elements f, g ∈ F , we can define the rth S-polynomial  −1 (r) (r) (r) (r) lcm(uf , ug ) lcm(uf , ug ) of f and g as the element Sr (f, g) = (lc (f )) f r (r) (r) uf uf −1  (r) (r) (r) (r) lcm(uf , ug ) lcm(uf , ug ) (lc (g)) g. − r (r) (r) ug ug Prove the following two statements that lead to an algorithm of computation of Gr¨ obner bases of an inversive difference vector spaces. obner basis of an E-submodule N of F with 1. Let G = {g1 , . . . , gt } be a Gr¨ respect to the orders <1 , . . . ,

(i) f ∈ N if and only if f −−−−−−−−→ 0 . <1 ,<2 ,···
(ii) If f ∈ N and f is (<1 , <2 , · · ·

Sr (gi , gj ) −−−−−−−−−→ 0 for any gi , gj ∈ G.
Then G is a Gr¨ obner basis of N with respect to
224

CHAPTER 3. DIFFERENCE MODULES

of elements of a group) and also in the group ring theory. We shall see some applications of these results in Chapter 7. Let A be a commutative ring and let elements of a finitely generated commutative group G act on A as automorphisms of this ring. Then A is said to be a G-ring. If J is a subring (an ideal) of A such that g(J) ⊆ J for every g ∈ G, then J is called a G-subring (respectively, a G-ideal) of the G-ring A. By a prime G-ideal we mean a G-ideal which is prime in the usual sense. If A0 is a G-subring of A and S ⊆ A, then A0 {S}G (or simply A0 {S} if the group G is fixed) denotes the smallest G-subring of A containing A0 and S (this is the intersection of all G-subrings of A containing A0 and S). In this case we say that A = A0 {S}G is the G-ring extension of A0 generated by S, and S is called the set of G-generators of A over A0 . Clearly, A0 {S}G = A0 [{g(η) | g ∈ G, η ∈ S}]. If S = {η1 , . . . , ηs }, we write A = A0 {η1 , . . . , ηs }G (or A = A0 {η1 , . . . , ηs } if G is fixed) and say that A0 is a finitely generated G-ring extension of A0 with the set of G-generators {η1 , . . . , ηs }. Let A and B be G-rings. A ring homomorphism φ : A → B is called a G-homomorphism if φ(g(a)) = g(φ(a)) for any a ∈ A, g ∈ G. The concepts of G-epimorphism, G-monomorphism, G-isomorphism, etc. are introduced in the usual way. Clearly, if φ : A → B is a G-homomorphism, then Ker A is a Gideal of A and the natural isomorphism A/Ker φ ∼ = φ(B) is a G-isomorphism. Also, if J is a G-ideal of A, then the natural ring epimorphism A → A/J is a G-epimorphism. If a G-ring is a field, it is called a G-field. It is easy to see that if a G-ring A is an integral domain, then the corresponding quotient field Q(A) can be treated g(a) a for every as a G-field (called a quotient G-field of A) such that g( ) = b g(b) a ∈ A, 0 = b ∈ A, g ∈ G. If K is a subfield of a G-field L such that g(K) ⊆ K for every g ∈ G, then K is said to be a G-subfield of L, and L is called a G-overfield or G-field extension of K (we also say that L/K is a G-field extension). If S ⊆ L, then the smallest G-subfield of L containing K and S (that is, the intersection of all G-subfields of L containing K and S) is denoted by KSG (or KS if the group G is fixed) and called the G-field extension of K generated by S; the set S is called the set of G-generators of KS over K. Obviously, KS coincides with the field K({g(η) | g ∈ G, η ∈ S}). If S is finite, S = {η1 , . . . , ηs }, and L = KS, we write L = Kη1 , . . . , ηs G (or L = Kη1 , . . . , ηs ) and say that L is a finitely generated G-field extension of K (or that the G-field extension L/K is finitely generated). Exercise 3.5.43 Let A be a G-ring and S a multiplicative subset of A such that g(S) ⊆ S for every g ∈ G (hence g(S) = S for all g ∈ G). Prove that the g(a) a for ring of quotients S −1 A can be treated as a G-ring such that g( ) = s g(s) all g ∈ G, a ∈ A, s ∈ S. (Show that this action of G on S −1 A is well-defined and S −1 A is a G-ring with respect to this action of G.) This ring is called the G-ring of quotients of A over S.

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

225

Let A be a G-ring and let a primary decomposition of G into a direct product of cyclic subgroups be fixed: G = {α1 }∞ × . . . {αn }∞ × {β1 }q1 × · · · × {βm }qm .

(3.5.24)

Then every element g ∈ G has a unique representation of the form lm g = α1k1 . . . αnkn β1l1 . . . βm

(3.5.25)

, lj ∈ Z and where ki m0 ≤ lj ≤ qj − 1 (1 ≤ i ≤ n, 1 ≤ j ≤ m). The number n ord g = i=1 |ki | + j=1 lj is called the order of the element g. It is easy to see that ord g ≥ 0, ord g = 0 if and only if g = 1 (the identity of the group G), and ord(g1 g2 ) ≤ ord g1 + ord g2 for any g1 , g2 ∈ G. Furthermore, for any r ∈ N, we set G(r) = {g ∈ G | ord g ≤ r}.  An expression of the form g∈G ag g, where ag ∈ A and only finitelymany ag are distinct from zero, is called a G-operator over A. Two G-operators g∈G ag g  and g∈G bg g are considered to be equal if and only if ag = bg for all g ∈ G. The set of all G operators over A has a natural structure of a left A-module. This A-module becomes a ring if for any elements g1 , g2 ∈ G, treated as G-operators, we define their product g1 g2 as it is defined in G, set ga = g(a)g for any g ∈ G, a ∈ A and expand these rules to the product of any two G-operators by distributivity. The ring we obtain is called the ring of G-operators over A or the ring of G-A-operators; it is denoted by FA or simply by F. The ring A and the group G are naturally considered as a subring and a subset of F , respectively (we use the same symbol 1 for the unity of A, the identity of G, and the unity of F). Note that F is a twisted group ring of the group G over A if one uses the ring theory terminology.  The order of a G-operator P = g∈G ag g is defined as the number ord P = max{ord g | ag = 0}. Clearly, for any two G operators P and Q, one has ord(P Q) ≤ ord P + ord Q and ord(P + Q) ≤ max{ord P, ord Q}. Setting Fr = {P ∈ F | ord P ≤ r} for any r ∈ N and Fr = 0 for any r ∈ Z, r < 0, we obtain an ascending filtration (Fr )r∈Z of the ring F called the standard filtration of F associated with decomposition (3.5.24). Clearly, Fr Fs = Fr+s for every r, s ∈ N. In what follows, while considering F as a filtered ring we always mean this filtration of F. Definition 3.5.44 With the above notation, a left F-module is called a G-Amodule or a G-module over the G-ring A. Thus, a G-A-module is a left A-module M such that the elements of group G act on M as endomorphisms of the additive group of M satisfying the following conditions: (i) The identity of G acts as the identity automorphism of M ; (ii) g1 (g2 (x)) = g2 (g1 (x)) = (g1 g2 )(x) for any g1 , g2 ∈ G, x ∈ M ; (iii) g(ax) = g(a)g(x) for any g ∈ G, a ∈ A, x ∈ M . If M and N are two G-A-modules, then a homomorphism of A-modules φ : M → N is called a G-homomorphism (or a G-A-homomorphism or a homomorphism of G-A-modules) if φ(g(x)) = g(φ(x)) for any g ∈ G, x ∈ M . The notions

CHAPTER 3. DIFFERENCE MODULES

226

of G-epimorphism, G-monomorphism, G-isomorphism, etc. of G-A-modules are defined in the usual way. We say that a G-A-module is finitely generated if it is finitely generated as a left F-module. By a filtration of a G-A-module M we mean a discrete and exhaustive ascending filtration of M as module over the filtered ring F. If M has such a filtration, it is called a filtered G-A-module. A filtration (Mr )r∈Z of a G-A-module M is called finite if every A-module Mr is finitely generated. This filtration is called good if there exists r0 ∈ Z such that Fs Mr = Mr+s for all integers r ≥ r0 and s ≥ 0. A finite and good filtration is called excellent. One can easily see that if A is a G-field and M is a s s   Fxi , then ( Fr xi )r∈Z is an excellent finitely generated G-A-module, M = i=1

i=1

filtration of M . Exercise 3.5.45 With the above notation, prove that if the ring A is Noetherian, then the ring F is left and right Noetherian. Exercise 3.5.46 Let M and N be two G-A-modules. Introduce the structure of  a G-A-module on each of the A-modules M A N and HomA (M, N ) (mimic the construction considered before Lemma 3.4.4) and prove the analogs of Lemmas 3.4.4 and 3.4.5. Then prove an analog of Theorem 3.4.6 in this case. Theorem 3.5.47 Let A be an Artinian G-ring and let (Mr )r∈Z be an excellent filtration of a G-A-module M (associated with the fixed decomposition (3.5.24) of the group G). Then there exists a numerical polynomial θ(t) in one variable t such that (i) θ(r) = lA (Mr ) for all sufficiently large r ∈ Z ; (ii) deg χ(t) ≤ n (= rank G) and the polynomial θ(t) can be represented in the form 

n  t+i (3.5.26) ai θ(t) = i i=0 where a0 , . . . , an ∈ Z and 2n |an . PROOF. Let gr F denote the graded ring associated with the filtered ring of G-operators F over A. Then the ring gr F is generated over A by the pairwise commuting elements x1 , . . . , x2n , y1 , . . . , ym which are the canonical images in gr F of the elements α1 , . . . , αn , α1−1 , . . . , αn−1 , β1 , . . . , βm , respectively. Furthermore, xi a = αi (a)xi , xn+i a = αi−1 (a)xn+i (1 ≤ i ≤ n) and yj a = βj (a)yj (1 ≤ j ≤ m) for any a ∈ A. A homogeneous component grs F (s ∈ N) of the graded ring gr F is an A-module generated by all monomials lm xki11 . . . xkinn y1l1 . . . ym

such that (a) ki , lj ∈ N, lj < qj (1 ≤ i ≤ n, 1 ≤ j ≤ m);

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS (b)

n 

ki +

i=1

m 

227

li = s;

j=1

(c) 1 ≤ i1 , . . . , in ≤ 2n and iµ − iν = n or 0 whenever µ = ν. In what follows we denote the ring gr F by Rn , while Rn and Rn will denote the the graded subrings of Rn whose elements can be written as linear combinations over A of the monomials free of xn and x2n , respectively.  Let gr M = s∈Z grs M be the graded gr F-module associated with the excellent filtration (Mr )r∈Z . In what follows, the module gr M will be also denoted by M and its homogeneous components grs M = Ms /Ms−1 (s ∈ Z) will be denoted by M (s) . Repeating the arguments of the proof of Theorem 3.2.3 (also used in the proof of Theorem 3.5.2), we obtain that gr M is a finitely generated gr F-module. Furthermore, as in the proof of Theorem 3.5.2, one can see that in order to prove our statement we need to prove the existence of a numerical polynomial h(t) in one variable t with the following properties: (a) h(s) = lA (M (s) ) for all sufficiently large s ∈ Z; (b) deg h(t) ≤ n − 1 and the polynomial f (t) can be represented as h(t) = n−1  t + i where b0 , . . . , bn−1 ∈ Z and 2n |bn−1 . bi i i=0 We shall prove the existence of the polynomial h(t) by induction on n. If n = 0, then M = gr M is a finitely generated A-module, hence one can take h(t) = 0. Suppose that n > 0 and let us consider the exact sequence of finitely generated R{x1 , . . . , x2n }-modules π

n 0 → Ker πn → M −−→ xn M → 0

where πn is the mapping of multiplication by xn . Clearly πn is an additive mapping and πn (ay) = αn (a)πn (y) for any y ∈ M, a ∈ A. It follows from Proposition 3.1.4 (with K = (Ker πn )(s) , M = M (s) , N = (xn M )(s) , and δ = πn ) that lA ((Ker πn )(s) ) + lA ((xn M)(s) ) = lA (M (s) )

(3.5.27)

for every s ∈ Z (we denote the sth homogeneous component of a graded module P by P (s) .) Since Ker πn and xn M are annihilated by the elements xn and x2n , respectively, we can treat Ker πn as a graded Rn -module and xn M as a graded Rn module.  By the inductive hypothesis, for any finitely generated Rn−1 -module N = p∈Z N (p) , there exists a numerical polynomial φN (t) such that φN (p) = lA (N (p) ) for all sufficiently large p ∈ Z, deg φN ≤ n − 2, and the polynomial n−2  t + i where c0 , . . . , cn−2 ∈ Z and ci φN (t) can be written as φN (t) = i i=0 2n−1 |cn−2 .

CHAPTER 3. DIFFERENCE MODULES

228

 If L = q∈Z L(q) is a finitely generated graded Rn -module, then the first and the last terms of the exact sequence of R-modules 2n → L(q+1) → L(q+1) /x2n L(q) → 0 0 → (Ker π2n )(q) → L(q) −−

π

(π2n is the mapping of multiplication by x2n ) are homogeneous components of finitely generated graded Rn−1 -modules. By the inductive hypothesis, there exists n−2  t + i (d0 , . . . , dn−2 ∈Z di a numerical polynomial ψL (t) of the form ψL (t)= i i=0 and 2n−1 |dn−2 ) such that ψL (q) = lA ((L(q+1) ) − lA (L(q) ) for all sufficiently large q ∈ Z, say, for all q ≥ q0 for some integer q0 . Since lA (L(q) ) = lA (L(q0 ) ) +

q  

lA (L(s) ) − lA (L(s−1) ) ,

s=q0 +1

it follows from Corollary 1.4.7 that there exists a numerical polynomial χL (t) such that χL (q) = lA (L(q) ) for all sufficiently large q ∈ Z and the polynomial n−1  t + i where d0 , . . . , dn−1 ∈ Z and di χL (t) can be written as χL (t) = i i=0

2n−1 |dn−1 . Clearly, a similar statement is true for any finitely generated graded  module K = q∈Z K (q) over the ring Rn . n−1  t + i and h2 (t) = ai Thus, there exist numerical polynomials h1 (t) = i i=0 n−1  t + i (ai , ai ∈ Z for i = 0, . . . , n − 1 and 2n−1 |an−1 , 2n−1 |an−1 ) such ai i i=0

that h1 (s) = lA ((Ker πn )(s) ) and h2 (s) = lA ((xn M)(s) ). Now we can use the arguments of the corresponding part of the proof of Theorem 3.5.2 to obtain that lA ((xi M)(s) ) = lA ((xn+i M)(s) ) for any s ∈ Z, i = 1, . . . , n. The last equality, together with equality (3.5.27), implies that lA (M (s) ) = h1 (s) + h2 (s) for all sufficiently large s ∈ Z. The last term of the sequence of graded Rn -modules 0 → x2n M → Ker πn → Ker πn /x2n M is a finitely generated graded Rn−1 -module. By the inductive hypothesis, there exists h3 (t) of degree at most n − 2 such that h3 (s) = ! a numerical polynomial " (s) for all sufficiently large s ∈ Z. It follows that an−1 = lA (Ker πn /x2n M) n−1  t + i  , where bn−1 = an−1 , hence the polynomial h(t) = h1 (t) + h2 (t) = bi i i=0 2an−1 is divisible by 2n , satisfies conditions (a) and (b). This completes the proof of the theorem.  Definition 3.5.48 Let A be an Artinian G-ring and let (Mr )r∈Z be an excellent filtration of a G-A-module M . Then the polynomial θ(t) whose existence

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

229

is established by Theorem 3.5.47 is called the G-dimension polynomial of M associated with the excellent filtration (Mr )r∈Z . Example 3.5.49 Let A be a G-field and let decomposition (3.5.24) of the group G be fixed. Let θF (t) denote the G-dimension polynomial of the corresponding ring of G-operators associated with the excellent filtration (Fr )r∈Z . Then lm θF (r) = dimA Fr = Card{α1k1 . . . αnkn β1l1 . . . βm |ki , lj ∈ Z, 0 ≤ lj < qj n 

(1 ≤ i ≤ n, 1 ≤ j ≤ m),

|ki | +

i=1

m 

lj ≤ r}

j=1

m for all sufficiently large r ∈ Z. Let Q = j=1 (qj − 1) and, for any s ∈ N, lm let φ1 (s) denote the number of distinct elements β1l1 . . . βm with 0 ≤ lj < qj m (j = 1, . . . , m) and j=1 lj = s, and let φ1 (s) denote the number of distinct n elements α1k1 . . . αnkn with k1 , . . . , kn ∈ Z and i=1 |ki | ≤ s. Clearly, θF (r) =

Q 

φ1 (i)φ2 (s − i)

i=0

for all sufficiently large s ∈ N, hence we can find θF (t) if we can find formulas for φ1 (s) and φ2 (s). Applying formulas (1.4.17) and (1.4.18) we obtain the following expressions for φ1 (s) and φ2 (s): φ2 (s) =

n 

(−1)n−i 2i

i=0



 n s+i , i i

and

φ1 (s) =

m+s−1 m−1

 =

m 

1≤j1 <···<jk ≤m qj1 +···+qjk ≤s

i=1

 m + s − qj1 − · · · − qjk − 1 . m−1



(−1)k

Therefore, θF (t) =

 Q   m+i−1 i=0 m 

+

m−1 (−1)k

k=1

 1≤j1 <···<jk ≤m qj1 +···+qjk ≤i

m + t − qj1 − · · · − qjk − 1 m−1

 

 n n t−i+l . × (−1)n−l 2l l l



(3.5.28)

l=0

Exercise 3.5.50 Formulate and proof analogs of Lemmas 3.5.5 and 3.5.6 for G-modules. Then prove an analog of Theorem 3.5.7 for this case.

CHAPTER 3. DIFFERENCE MODULES

230

Let K be a G-field and let (Fr )r∈Z be the standard filtration of the ring of G-K-operators associated with decomposition (3.5.24). Let M be a finitely l  Fr zi )r∈Z generated G-K-module with generators z1 , . . . , zl and let (Mr = i=1

be the corresponding excellent filtration of M . Then the field K can be also treated as an inversive difference field with the basic set σ = {α1 , . . . , αn } and M can be considered as a finitely generated σ ∗ -K-module (i. e., module over the ring of σ ∗ -operators E over K) with the set of generators {hzi | h ∈ H, 1 ≤ i ≤ l} where H denotes the (finite) periodical part of the group G (H = {g ∈ G | g k = 1 l   for some k ∈ N}). Clearly, (Mr = Er hzi )r∈Z is an excellent filtration i=1 h∈H

of the σ ∗ -K-module M , so we can consider the σ ∗ -dimension polynomial χ(t) 

n  t+i be the G-dimension ai associated with this filtration. Let θ(t) = i i=0 polynomial of M associated with the filtration (Mr )r∈Z . Since ord h ≤ Q = m (q − 1) for every h ∈ H, one has Mr ⊆ Mr ⊆ Mr+Q for every r ∈ Z. j j=1 Therefore, θ(r) ≤ χ(r) ≤ θ(r + Q) for all sufficiently large r ∈ Z. It follows an that if d = deg θ(t), then d = σ ∗ -typeK M , n = σ ∗ -dimK M , the coefficient ad 2 ad is divisible by 2d , and d = σ ∗ -tdimK M . Thus, the degree d of θ(t) and the 2 ad an numbers n and d , do not depend on the choice of an excellent filtration of 2 2 M associated with decomposition (3.5.24). Let us show that these numbers do not depend on the decomposition of G either. Indeed, let G = {γ1 }∞ × . . . {γn }∞ × {δ1 }q1 × · · · × {δm }qm be another primary decomposition of G and let lim αi = γ1ki1 . . . γnkin δ1li1 . . . δm (1 ≤ i ≤ n), u

v

vjm γj = α1 j1 . . . αnujn β1 j1 . . . βm (1 ≤ i ≤ n)

where kiµ , liν , uiλ , vjp ∈ Z, 0 ≤ liν < qν , 0 ≤ vjp < qp (1 ≤ µ, λ ≤ n, 1 ≤ ν, p ≤ m). Setting q = q1 . . . qm we obtain that αiq = (γ1q )ki1 . . . (γnq )kin and γjq = (α1q )uj1 . . . (αnq )ujn (1 ≤ i, j ≤ n). Let σ1 = {α1q , . . . , αnq } and σ2 = {γ1q , . . . , γnq }. Then the ring of σ1∗ -operators over K (when K is treated as a σ1∗ -field) coincides with the ring of σ2∗ -operators over K (when K is considered as a σ2∗ -field). Let us denote this ring by E  and consider the standard filtration (Er )r∈Z of E  as a ring of σ1∗ -operators. It is easy to see that the elements hα1k1 . . . αnkn zj (h ∈ H, 0 ≤ k1 , . . . , kn < q, 1 ≤ j ≤ l) generate M over E. Let ˆr = M

l  



j=1 h∈H 0≤k1 ,...,kn
Er hα1k1 . . . αnkn zj

3.5. σ ∗ -DIMENSION POLYNOMIALS AND THEIR INVARIANTS

231

ˆ r )r∈Z is an excellent filtration of the σ ∗ -K-module M . Since (r ∈ Z). Then (M for any element g = α1qd1 +e1 . . . αnqdn +en (0 ≤ ei < q for i = 1, . . . , n), one has n n   ord g = |qdi + ei | ≥ ord h + q( |di | − n), the inequality ord g ≤ qr (r ∈ N) i=1

implies that

n 

i=1

ˆ r+n for all sufficiently |di | ≤ r + n. It follows that Mqr ⊆ M

i=1

ˆ r ⊆ Mqr+qn+Q , the degree of the G-dimension large r ∈ Z. Since we also have M polynomial θ(t) is equal to the degree d of a σ1∗ -dimension polynomial of M , and ad σ ∗ -dimK M σ ∗ -dimK M an = 2 n and the number n and d are equal to the integers 1 n 2 2 q q ∗ ∗ σ -tdimK M σ1 -tdimK M = 2 , respectively. We obtain the following theorem. d q qd Theorem 3.5.51 Let K be a G-field, let M be a finitely G-K-module, and let 

n  t+i be the G-dimension polynomial of M associated with a θ(t) = ai i i=0 decomposition of the form (3.5.24) and an excellent filtration of M . Then the an ad integers d = deg θ(t), a = n , and a(d) = d are independent of the choice of 2 2 such decomposition and filtration. If σ is a set of free generators of some free commutative subgroup H of G whose rank is equal to the rank of G, then d = σ ∗ -typeK M , a = σ ∗ -dimK M , and a(d) = σ ∗ -tdimK M . Definition 3.5.52 With the notation of the last theorem, the integers d, a, and a(d) are called the G-dimension, G-type, and typical G-dimension of the G-Kmodule M , respectively. They are denoted, respectively, by δG(M ), tG(M ), and tδG(M ). The following two propositions can be easily proved using the arguments of the proofs of Theorems 3.2.11 and 3.2.12. Proposition 3.5.53 Let K be a G-field and let i

j

→M − → P −→ 0 0 −→ N − be an exact sequence of finitely generated G-K-modules. Then δG(N )+δ(GP ) = δG(M ). Proposition 3.5.54 Let K be a G-field, F the ring of G-operators over K, and M a finitely generated G-K-module. Let G0 be any free commutative subgroup of G such that rank G0 = rank G. Then δG(M ) is equal to the maximal number of elements x1 , . . . , xp ∈ M such that the set {g(xi ) | g ∈ G0 , 1 ≤ i ≤ p} is linearly independent over K.

232

3.6

CHAPTER 3. DIFFERENCE MODULES

Dimension of General Difference Modules

In this section we generalize the results on difference and inversive difference modules obtained in the preceding sections. We also describe the main invariants of characteristic polynomials of difference and inversive difference modules in the frame of the theory of Krull dimension. Let R be a difference ring whose basic set is a union of a set σ = {α1 , . . . , αm } of injective endomorphisms of the ring R and a set  = {β1 , . . . , βn } of automorphisms of R. (As usual, any two elements of the basic set σ ∪  commute with each other.) Let T (or Tσ ) and Γ (or Γ ) denote the free commutative semigroup generated by the set σ and the free commutative group generated by the set , respectively. Furthermore, let Θ denote the commutative semigroup of all power products of the form k m l1 β1 . . . βnln (3.6.1) θ = α1k1 . . . αm where k1 , . . . , km ∈ N and l1 , . . . , ln ∈ Z. This semigroup contains both the semigroup T and the group Γ. In what follows, the subset {β1 , . . . , βn , β1−1 , . . . , βn−1 } of Θ will be denoted by ∗ , and the ring R will be called a σ-∗ -ring. (As before, assigning ∗ to the set  means that we treat R as an inversive difference ring with respect to the basic set . At the same time, the endomorphisms αi are not supposed to be bijective, so we consider only power products (3.6.1) with positive exponents ki , even though some of the αi could be automorphisms of R.) By the orders of the element θ of the form (3.6.1) relative to the sets σ and m n   ki and ord θ = |lj |, respectively; the  we mean the numbers ordσ θ = i=1

j=1

number ord θ = ordσ θ + ord θ is called the order of θ. Furthermore, for every r ∈ N, Θ(r) will denote the set of all elements θ ∈ Θ such that ord θ ≤ r.  Definition 3.6.1 An expression of the form θ∈Θ aθ θ, where aθ ∈ R for any from zero, is called a σθ ∈ Θ and only finitely many elements a θ are different  ∗ -operator over R. Two σ-∗ -operators θ∈Θ aθ θ and θ∈Θ bθ θ are considered to be equal if and only if aθ = bθ for all θ ∈ Θ. The set of all σ-∗ -operators over a σ-∗ -ring R will be denoted by F. As in the case of difference or inversive difference operators, the setF can be   a θ + bθ θ = θ∈Θ (aθ + equipped with a ring structure if one sets θ θ∈Θ θ∈Θ      (aa )θ, a θ δ = a (θδ), δa = δ(a)δ for bθ )θ, a θ∈Θ a θθ = θ θ θ θ∈Θ  θ∈Θ θ∈Θ ∗ b θ ∈ F, a ∈ R, δ ∈ σ ∪  and extends this any elements θ∈Θ aθ θ, θ θ∈Θ operations to the whole set F by distributivity. The resulting ring is called the ring of σ-∗ -operators over R. (Note that the ring of σ-operators D and the ring of ∗ -operators E introduced in Sections 3.1 and 3.4, respectively, are subrings of the ring F.)  By the order of a σ-∗ -operator u = θ∈Θ aθ θ ∈ F we mean the nonnegative integer ord u = max{ord θ|aθ = 0}. If r ∈ N, then the set of all σ-∗ operators whose orders do not exceed r will be denoted by Fr . Setting Fr = 0 for

3.6. DIMENSION OF GENERAL DIFFERENCE MODULES

233

any negative integer r, we obtain a discrete and exhaustive ascending filtration (Fr )r∈Z of the ring F. This filtration is called the standard filtration of the ring of σ-∗ -operators F . In what follows, while considering F as a filtered ring we always mean that F is equipped with the standard filtration. Definition 3.6.2 Let R be a σ-∗ -ring and F the ring of σ-∗ -operators over R. Then a left F-module is called a σ-∗ -R-module. In other words, an R-module M is called a σ-∗ -R-module if the elements of the set σ ∪∗ act on M in such a way that δ(x + y) = δx + δy, δ1 (δ2 x) = δ2 (δ1 x), δ(ax) = δ(a)δx, and β(β −1 x) = x for any x, y ∈ M, a ∈ R, δ, δ1 , δ2 ∈ σ ∪ ∗ , and β ∈ ∗ . If R is a field then a σ-∗ -R-module is also called a vector σ-∗ -space over R or a vector σ-∗ -R-space It is easy to see that if R is a σ-∗ -ring, then any σ-∗ -R-module is at the same time a σ-R-module and an ∗ -R-module in the sense of definitions 3.1.2 and 3.4.3, respectively. In accordance with the definition of the difference homomorphism (see Definition 3.1.3), if M and N are two σ-∗ -R-modules, then a mapping f : M → N is called a σ-∗ -homomorphism if f is a homomorphism of R-modules such that f (δx) = δf (x) for any x ∈ M, δ ∈ σ ∪ . (It is easy to see that in this case f commutes with the elements β1−1 , . . . , βn−1 as well). An injective (respectively, surjective or bijective) σ-∗ -homomorphism is said to be a σ-∗ -monomorphism (respectively, a σ-∗ -epimorphism or a σ-∗ -isomorphism). Definition 3.6.3 Let R be a σ-∗ -ring and F the ring of σ-∗ -operators over R equipped with the standard filtration (Fr )r∈Z . An ascending chain ∗ of R-submodules (Mr )r∈Z of a σ-  -R-module M is called a filtration of M if Fs Mr ⊆ Mr+s for all r, s ∈ Z, Mr = M , and Mr = 0 for all sufficiently r∈Z

small r ∈ Z. (Thus, by a filtration of a σ-∗ -R-module M we mean an exhaustive and discrete filtration of M as a left F-module.) A filtration (Mr )r∈Z of a σ-∗ -R-module M is called excellent if every its component Mr (r ∈ Z) is a finitely generated R-module and there exists r0 ∈ Z such that Fs Mr = Mr+s for any r ∈ Z, r ≥ r0 , s ∈ N. Let R be a σ-∗ -ring and let M and N be filtered σ-∗ -R-modules with filtrations (Mr )r∈Z and (Nr )r∈Z , respectively. Then a σ-∗ -homomorphism f : M → N is said to be a σ-∗ -homomorphism of filtered σ-∗ -R-modules if f (Mr ) ⊆ Nr for any r ∈ Z. It is easy to see that if a σ-∗ -R-module M has an excellent filtration, then M is a finitely generated F-module. In what follows, a finitely generated F-module will be also called a finitely generated σ-∗ -R-module. of generators of a σIf R is a σ-∗ -field and {z1 , . . . , zq } is a finite   system n n   Fzi ), then Fr z i is an excellent ∗ -R-module M (that is M = i=1

i=1

r∈Z

filtration of M called the standard filtration of M associated with the system of generators {z1 , . . . , zq }. Thus, every vector σ-∗ -space over a σ-∗ -field has excellent filtrations.

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234

The following result generalizes theorems 3.2.3 and 3.5.2 that introduce characteristic polynomials of difference and inversive difference modules. Theorem 3.6.4 Let R be an Artinian σ-∗ -ring with a basic set σ ∪  where σ = {α1 , . . . , αm } and  = {β1 , . . . , βn } are the sets of injective endomorphisms and automorphisms of the ring R, respectively. Let M be a σ-∗ -R-module and let (Mr )r∈Z be an excellent filtration of M . Then there exists a numerical polynomial Φ(t) in one variable t such that (i) Φ(r) = lR (Mr ) for all sufficiently large r ∈ Z (ii) deg Φ(t) ≤ m + n and the polynomial Φ(t) can be represented as

Φ(t) =

m−1  i=0

ai

 

 n t+i t+m+j + . 2j bj i m+j j=0

(3.6.2)

where a0 , . . . , am−1 , b0 , . . . , bn ∈ Z. PROOF. Let us consider the graded ring gr F associated with the filtration (Fr )r∈Z of the ring of σ-∗ -operators F . Let x1 , . . . , xm , y1 , . . . , yn , yn+1 , . . . , y2n be the canonical images of the elements α1 , . . . , αm , β1 , . . . , βn , β1−1 , . . . , βn−1 , respectively, in the ring gr F, so that xi = αi + F0 ∈ gr1 F = F1 /F0 yj = βj + F0 ∈ gr1 F, and yn+j = βj−1 + F0 ∈ gr1 F (1 ≤ i ≤ m, 1 ≤ j ≤ n). It is easy to see that x1 , . . . , xm , y1 , . . . , y2n generate the ring gr F over R, these elements commute with each other and xi a = axi , yj a = βj (a)yj , yn+j a = βj−1 (a)yn+j for i = 1, . . . , m; j = 1, . . . , n. In the rest of the proof the ring gr F will be also denoted by R{x1 , . . . , xm , y1 , . . . , y2n }. Note that a homogeneous component generatedby the set of all power products grs F (s ∈ N) is the R-module m n xk11 . . . xkmm yil11 . . . yilnn such that µ=1 kµ + ν=1 lν = s (l1 , . . . , ln are distinct integers from the set {1, . . . , 2n} such that lp − lq = n for any p = 1, . . . , n; q = 1, . . . , n).  Let gr M = r∈Z grr M be the graded gr F-module associated with the excellent filtration (Mr )r∈Z (so that grr M = Mr /Mr−1 for any r ∈ Z). Repeating the arguments of the proof of Theorem 3.2.3, one  can obtain that gr M is a finitely generated gr F-module. Since lR (Mr ) = s≤r lR (grs M ), the theorem will be proved if one can prove the existence of a numerical polynomial g(t) in one variable t such that (a) g(s) = lR (grs M ) for all sufficiently large s ∈ Z; (b) deg g(t) ≤ m + n − 1 and the polynomial g(t) can be represented as g(t) =

m−1  i=0

 n−1 

 t+i t+m+j j+1 + ai 2 bj i m+j j=0

where a0 , . . . , am−1 , b0 , . . . , bn−1 ∈ Z. (Indeed, if such a polynomial g(t) exists, then Corollary 1.4.7 implies the existence of the polynomial Φ(t) with the desired properties.) We shall prove

3.6. DIMENSION OF GENERAL DIFFERENCE MODULES

235

the existence of the polynomial g(t) with the properties (a) and (b) by induction on n = Card . If n = 0, then gr M is a finitely generated graded module over the Artinian σ-ring R. In this case, the existence of the polynomial g(t) with the desired properties is stated by Theorem 3.2.1. Now, suppose that n > 0. Let us set M = grM and consider the exact sequence of finitely generated R{x1 , . . . , xm , y1 , . . . , y2n }-modules θ

n 0 → Ker θn → M −→ yn M → 0

(3.6.3)

where θn is the mapping of multiplication by yn , that is θn (z) = yn z for any z ∈ M. It is easy to see that θn is an additive mapping and θn (az) = αn (a)θn (z) for any z ∈ M, a ∈ R. Repeating the arguments of the proof of Theorem 3.5.2 we obtain that there exist numerical polynomi

m−1 m−1  t + i n−1   t + i  j  t+m+j + and g2 (t) = + ai 2 bj ai als g1 (t) = i m + j i i=0 j=0 i=0 

n−1  t+m+j (ai , ai , bj , bj ∈ Z for i = 0, . . . , m−1; j = 0, . . . , n−1) such 2j bj m + j j=0  that g1 (s) = lR (Ker θn grs M ) and g2 (s) = lR (θn (grs M )) for all sufficiently large s ∈ Z. Furthermore, as in the proof of Theorem 3.5.2, we obtain that there

 m−1  t + i n−2  t+m+j + ci 2j+1 dj exist a numerical polynomial g3 (t) = i m+j i=0 j=0 (ci , dj ∈ Z for i = 0, . . . , m − 1; j = 0, . . . , n − 2) such that g2 (t) = g1 (t)−g3 (t). (The polynomial g3 (t) is characterized by the property that g3 (s) = lR ((Kerθn |grs M )/y2n grs−1 M )) for all sufficiently large s ∈ Z.) Since the function lR (·) is additive in the class of all finitely generated R-modules, the exact sequence (3.6.3) implies that the polynomial g(t) = g1 (t)+g2 (t) = 2g1 (t)−g3 (t) =   n−2 

m−1   t+m+n−1 t+i t+m+j  j+1  n  + + 2 bn−1 (2ai − ci ) 2 (bj − dj ) m+n−1 i m+j i=0 j=0 satisfies conditions (a) and (b). This completes the proof of the theorem.



Definition 3.6.5 Let R be an Artinian σ-∗ -ring with a basic set σ ∪  where σ = {α1 , . . . , αm } and  = {β1 , . . . , βn } are the sets of injective endomorphisms and automorphisms of R, respectively. Let M be a σ ∗ -R-module and let (Mr )r∈Z be an excellent filtration of a M . Then the polynomial Φ(t) whose existence is established by Theorem 3.6.4 is called the σ-∗ -dimension or characteristic polynomial of the module M associated with the excellent filtration (Mr )r∈Z . Example 3.6.6 Let K be a σ-∗ -field with a basic set σ ∪  where σ = {α1 , . . . , αn } and  = {β1 , . . . , βn } are the sets of injective endomorphisms and automorphisms of the field K, respectively. Then the ring of σ-∗ -operators F over K can be considered as a σ-∗ -K-module equipped with the excellent filtration

CHAPTER 3. DIFFERENCE MODULES

236

(Fr )r∈Z considered above. Let ΦF (t) be the σ-∗ -dimension polynomial of F associated with this filtration. Then k m l1 β . . . βnln ∈ Θ| ΦF (r) = dimR (Fr ) = Card {α1k1 . . . αm

m 

ki +

i=1

n 

|lj | ≤ r}

j=1

for all sufficiently large r ∈ Z. In other words, for all sufficiently large r ∈ Z, ΦF (r) is equal to the number of (m + n)-tuples in the set A = m n   ki + |lj | ≤ r}. We shall find this {(k1 , . . . , km , l1 , . . . , ln ) ∈ Nm × Zn | i=1

j=1

number by decomposing A into the disjoint union of r + 1 sets Aq (0 ≤ q ≤ r) m n   such that Aq = {(k1 , . . . , km , l1 , . . . , ln ) ∈ Nm ×Zn | ki = q, |lj | ≤ r −q}. i=1

j=1

  n



r−q+i q+m−1  n−i i n By Proposition 1.4.9, Card Aq = (−1) 2 i i m−1 i=0   

r n n  q+m−1   n r−q+i = hence Card A = (−1)n−i 2i (−1)n−i 2i i m − 1 i q=0 i=0 i=0   r n   q+m−1 r−q+i . i i m−1 q=0 The last expression can be essentially simplified if one notices that    r  m+r+i q+m−1 r−q+i . = m+i i m−1 q=0 

m+r+i = Indeed, applying formulas (1.4.13) and (1.4.16), we obtain that m+i m+i r   Card {(x1 , . . . , xm+i ∈ Nm+i | xk ≤ r} = Card {(x1 , . . . , xm+i ∈ k=1

Nm+i |

m  k=1

xk = q and

m+i  k=m+1

xk ≤ r − q} =

q=0

  r  q+m−1 r−q+i q=0

m−1

i

.

It follows that the σ-∗ -dimension polynomial ΦF (t) can be expressed in the form 

 n  n t+m+i . (3.6.4) ΦF (t) = (−1)n−i 2i m+i i i=0 Let R be a σ-∗ -ring with a basic set σ ∪  where σ = {α1 , . . . , αn } and  = {β1 , . . . , βn } are the sets of injective endomorphisms and automorphisms, R, F[x] the respectively. Let F be the ring of σ-∗ -operators over   ring of polynomialsin one variable x over F, and F  the subring r∈N Fr R Rxr of the σ-∗ -R-module ring F R R[x]. Furthermore, for any finitely generated  M with   a filtration (Mr )r∈Z , let M denote the left F -module r∈N Mr R Rxr .

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237

The proofs of the following two lemmas and Theorem 3.6.9 are similar to the proofs of the corresponding statements for difference and inversive difference modules (see Lemmas 3.2.6, 3.2.7, 3.5.5, 3.5.6 and Theorems 3.2.8, 3.5.7). Lemma 3.6.7 Let R be a σ-∗ -ring and let (Mr )r∈Z be a filtration of a σ-∗ R-module M such that all its components Mr (r ∈ Z) are finitely generated R-modules. Then the following conditions are equivalent: (i) The filtration (Mr )r∈Z is excellent; (ii) M  is a finitely generated F  -module. Lemma 3.6.8 Let R be a Noetherian σ-∗ -ring such that elements of the set σ are automorphisms of R. Then the ring F  is left Noetherian. Theorem 3.6.9 Let R be a Noetherian σ-∗ -ring such that elements of the set σ are automorphisms of R. Let ρ : N −→ M be an injective homomorphism of filtered σ-∗ -R-modules and let the filtration of the module M be excellent. Then the filtration of N is also excellent. Let R be a σ-∗ -field, M a finitely generated σ-∗ -R-module, and (Mr )r∈Z and (Mr )r∈Z two excellent filtrations of M . As in the case of difference and inversive difference modules, one can easily see that there exists q ∈ N such  for all suffisiently large r ∈ Z. Therefore, if that Mr ⊆ Mr+q and Mr ⊆ Mr+q ∗ Φ(t) and Φ1 (t) are σ- -dimension polynomials associated with the filtrations (Mr )r∈Z and (Mr )r∈Z , respectively, then Φ(r) ≤ Φ1 (r +q) and Φ1 (r) ≤ Φ(r +q) for all sufficiently large r ∈ Z. We arrive at the following statement. Theorem 3.6.10 Let R be a σ-∗ -field, M a finitely generated σ-∗ -R-module, 

m−1 n  t + i  t+m+j + the σ-∗ -dimension polynomial ai 2j bj and Φ(t) = i m + j i=0 j=0 of M associated with some excellent filtration of this module (ai , bj ∈ Z for ∆m+n Φ(t) 0 ≤ i ≤ m − 1, 0 ≤ j ≤ n). Then the integers bn = , d = deg Φ(t), 2n and ⎧ d ∆ Φ(t) ⎪ ⎨ if d ≥ m 2d−m cd = ⎪ ⎩ d ∆ Φ(t) if d < m do not depend on the choice of the excellent filtration the polynomial Φ(t) is associated with. Definition 3.6.11 Let R be a σ-∗ -field, M a finitely generated σ-∗ -R-module, and Φ(t) the σ-∗ -dimension polynomial of M associated with an excellent fil∆m+n Φ(t) tration of M . Then the integers , d = deg Φ(t), and cd considered in 2n ∗ Theorem 3.6.10 are called σ- -dimension, σ-∗ -type, and typical σ-∗ -dimension of the module M . These numbers are denoted by σ∗ dimR M , σ∗ typeR M , and tσ∗ dimR M , respectively.

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Exercise 3.6.12 Let R be a σ-∗ -field with a basic set σ ∪ where σ = {α1 , α2 } and  = {β}. Let M be a σ-∗ -R-module with one generator z and the defining relation α1 βz = α2 z. (One can treat M as a σ-∗ -R-module E/F(α1 β − α2 )e where F is the ring of σ-∗ -operators over R and E is a free left F-module with a single free generator e.) Find the σ-∗ -dimension polynomial of the module M associated with the excellent filtration (Fr z)r∈Z . Determine σ∗ dimR M , σ∗ typeR M , and tσ∗ dimR M . Let R be a σ-∗ -field with a basic set σ ∪ , where σ = {α1 , . . . , αm } and  = {β1 , . . . , βn }, and let Θ be the commutative semigroup of all power products of the form (3.6.1). Elements z1 , . . . , zk of a σ-∗ -R-module M are said to be σ∗ -linearly independent over the field R if and only if the family {θzi |θ ∈ Θ, 1 ≤ i ≤ k} is linearly independent over R. If this family is linearly dependent over R, we say that z1 , . . . , zk are σ-∗ -linearly dependent over the field R. (It is easy to see that if F is the ring of σ-∗ -operators over the σ-∗ -field R, then elements z1 , . . . , zk are σ-∗ -linearly independent over R if and only if they are linearly independent over F.) The following two statements can be proven in the same way as the corresponding results on difference modules (see Theorems 3.2.11 and 3.2.12). Theorem 3.6.13 Let R be a σ-∗ -field such that elements of the set σ are j i →M − → P −→ 0 be an exact sequence automorphisms of R, and let 0 −→ N − ∗ of finitely generated σ- -R-modules ( i and j are σ-∗ -homomorphisms). Then σ∗ dimR M = σ∗ dimR N + σ∗ dimR P . Theorem 3.6.14 Let R be a σ-∗ -field such that elements of the set σ are automorphisms of R, and let M be a finitely generated σ-∗ -R-module. Then σ∗ dimR M is equal to the maximal number of elements of M that are σ-∗ linearly independent over the field R. Exercises 3.6.15 Let K be a σ-∗ -field with a basic set δ = σ ∪  where σ = {α1 , . . . , αm } and  = {β1 , . . . , βn } are the sets of injective endomorphisms and automorphisms of the field K, respectively. Let T be the free commutative semigroup generated by σ, Γ the free commutative group generated by , and Θ the free commutative semigroup of elements of the form θ = α1k1 . . . αnkn β1l1 . . . βnln where ki ∈ N, lj ∈ Z (1 ≤ i ≤ m, 1 ≤ j ≤ n). mAs in θ = the beginning of this section, for any such an element θ, we set ord σ i=1 ki , n ord θ = j=1 |lj | and ord θ = ordσ θ + ord θ. Furthermore, for any r, s ∈ N, let T (r) = {θ ∈ Θ | ordσ θ ≤ r}, Γ(s) = {θ ∈ Θ | ord θ ≤ s} and Θ(r, s) = {θ ∈ Θ | ordσ θ ≤ r, ord θ ≤ s}. Let D, E, and F denote  the rings of σ-, -, and σ--operators over K, respectively, and for any A = θ∈Θ aθ θ ∈ F, let ordσ A = max{ordσ θ | aθ = 0} (σ) and ord A = max{ord θ | aθ = 0}. Furthermore, for any r ∈ Z, let Fr = {A ∈ () (σ) () F | ordσ A ≤ r} and Fr = {A ∈ F | ord A ≤ r} if r ≥ 0, and Fr = Fr = 0 if r < 0. Let M be a σ--K-module, that is, a left F-module (clearly, M is also a module over the rings D and E). We say that M is σ- (-) filtered if M is a filtered

3.6. DIMENSION OF GENERAL DIFFERENCE MODULES

239 (σ)

module over the ring F treated as a filtered ring with the filtration (Fr )r∈Z () (respectively, with the filtration (Fr )r∈Z ). The corresponding filtration of M (called, respectively, a σ- or an -filtration) is supposed to be exhaustive an (σ) (σ) separated. If (Mr )r∈Z is a σ-filtration of M such that every component Mr (σ) (σ) is a finitely generated E-module and there exists r0 ∈ Z such that Fs Mr = (σ) Mr+s for all r ≥ r0 , s ≥ 0, then this σ-filtration is called excellent. Similarly, an () () -filtration (Mr )r∈Z is said to be excellent if every component Mr is a finitely () () () generated D-module and there exists r0 ∈ Z such that Fs Mr = Mr+s for all r ≥ r0 , s ≥ 0. 1. Prove that if a σ--K-module M is finitely generated as a left F-module k k   Fr(σ) xj )j∈Z and ( Fr() xj )j∈Z are excellent by elements x1 , . . . , xk , then ( j=1

j=1

σ- and -filtration of M , respectively. (σ)

2. Prove that if (Mr )r∈Z is an excellent σ-filtration of a σ--K-module M , then there exists a numerical polynomial φσ (t) with the following properties: (σ)

(i) φσ (r) = ∗ -dimK Mr for all sufficiently large r ∈ N. (ii) deg φσ ≤ m, so the polynomial φσ (t) can be written in the form φσ (t) = 

m  t+i with integer coefficients aσi . aσi i i=0 (iii) aσm = σ∗ dimK M . ()

3. Prove that if (Mr )r∈Z is an excellent -filtration of a σ--K-module M , then there exists a numerical polynomial φ (t) with the following properties: ()

(i) φ (r) = σ-dimK Mr for all sufficiently large r ∈ N. (ii) deg φ ≤ n, and the polynomial φ (t) can be written in the form φ (t) = 

n  t+j with integer coefficients ai . 2j aj j j=0 (iii) an = σ∗ dimK M . 4. Generalize the Gr¨obner basis technique developed in section 3.3 and the technique of characteristic sets considered in section 3.5 to the case of free modules over the ring of σ--operators F and any fixed partition of the set of operators σ ∪ . Formulate and prove analogs of Theorems 3.3.16 and 3.5.38 for this case. Let A be a commutative ring, M an A-module and U a family of Asubmodules of M . Furthermore, let BU denote the set of all pairs (N, N  ) ∈ U ×U ¯ be the augmented set of integers, that is the set such that N ⊇ N  , and let Z ¯ is considered as a obtaining by adjoining a new symbol ∞ to the set Z (Z linearly ordered set whose order < is the extension of the natural ordering of Z such that a < ∞ for any a ∈ Z).

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CHAPTER 3. DIFFERENCE MODULES

Proposition 3.6.16 With the above notation, there exists a unique mapping ¯ with the following properties: µU : BU → Z

(i) µU (N, N  ) ≥ −1 for every pair (N, N  ) ∈ BU ; (ii) If d ∈ N, then µU (N, N  ) ≥ d if and only if N = N  and there exists an infinite chain N = N0 ⊇ N1 ⊇ · · · ⊇ N  such that µU (Ni−1 , Ni ) ≥ d − 1 for all i = 1, 2, . . . .

PROOF. Let us define the desired mapping µU as follows. If N ∈ U , we set µU (N, N ) = −1. If (N, N  ) ∈ BU , N = N  , and condition (ii) holds for all d = ∈ N, we set µU (N, N  ) = ∞. Finally, if a pair (N, N  ) ∈ BU satisfies condition (ii) for some d ∈ N, we define µU (N, N  ) as the greatest of such integers d. It is easy to see that the mapping µ is well-defined, it satisfies conditions (i) and (ii), and it is iniquely determined by these conditions.  ¯ satisfy Exercise 3.6.17 Let (N, N  ) ∈ BU and let the mapping µU : BU → Z conditions (i) and (ii) of Proposition 3.6.16. Prove the following properties of this mapping. (a) µU (N, N  ) = −1 if and only if N = N  . (b) µU (N, N  ) = 0 if and only if N  N  and any chain N = N0 ⊇ N1 ⊇ · · · ⊇ N  (Ni ∈ U for i = 0, 1, 2, . . . ) stabilizes at some stage (that is, there exists m ∈ N such that Nm = Nm+1 = · · · = N  ). (c) µU (N, N  ) ≥ 1 if and only if there exists an infinite strictly descending chain N = N0  N1  · · ·  N  with Ni ∈ U (i = 0, 1, 2, . . . ). Definition 3.6.18 With the above notation, the least upper bound of the set {µU (N, N  )|(N, N  ) ∈ BU } is called the type of the A-module M over the family of its submodules U . Definition 3.6.19 Let A be a ring, M an A-module, and U a family of A-submodules of M . Then the least upper bound of the lengths of all chains N0  N1  · · ·  Np , where Ni ∈ U and µU (Ni−1 , Ni ) = typeU M (i = 0, . . . , p and the number p is considered as the length of the chain), is called the dimension of the A-module M over the family U . The type and dimension of an A-module M over a family of its A-submodules U are denoted by typeU M and dimU M , respectively. Exercise 3.6.20 Let V be a finite-dimensional vector space over a field K and let U be the family of all vector K-subspaces of V . Show that typeU V = 0 and dimU V = dim V (where dim V denotes the dimension of the vector K-space V in the usual sense). Let M be a module over a ring A and let U be a family of A-submodules of M . It is easy to see that if typeU M < ∞, then dimU M ≥ 1. At the same time, if typeU M = ∞, then dimU M can be equal to zero. One can obtain the corresponding example by taking an infinite direct sum of vector spaces of dimensions 1, 2, . . . ; we leave the details to the reader as an exercise.

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241

Exercise 3.6.21 Let K be a field and V an infinite-dimensional vector K-space with a countable basis. Show that if U is the family of all vector K-subspaces of V , then typeU V = ∞ and dimU V = ∞. [Hint: Represent a countable basis B of the vector space V as a disjoint
∞ (1) countable union i=1 Bi of countable sets. Then consider vector subspaces Wk
k generated by the sets B \ i=1 Bi and show that µU (V, 0) = ∞.] Exercise 3.6.22 Let F be the ring of functions of one real variable t that are defined and continuous on the whole real line R. It is easy to see that for any two real numbers a and b, a < b, the set O(a, b) = {f (t) ∈ F |f (t) = 0 for any t ∈ R \ (a, b)} is an ideal of the ring F . Let U be the family of all such ideals. Find typeU F and dimU F . Exercise 3.6.23 Let A be a commutative ring of finite Krull dimension d and let U be the family of all prime ideals of A. Show that typeU A = 0 and dimU A = d. Theorem 3.6.24 Let R be a σ-∗ -field whose basic set is a union of a set σ = {α1 , . . . , αm } of injective endomorphisms of the field R and a set  = {β1 , . . . , βn } of automorphisms of R. Let M be a finitely generated σ-∗ -Rmodule and let U be the family of all σ-∗ -R-submodules of M . Then: (i) If σ∗ dimR M > 0, then typeU M = m + n and dimU M = σ∗ dimR M ; (ii) If σ∗ dimR M = 0, then typeU M < m + n. PROOF. As before, let Θ and F denote the commmutative semigroup of all power products of the form (3.6.1) and the ring of σ-∗ -operators over R, ∗ respectively. Let (Mr )r∈Z be an excellent  filtration of theσ- -R-module M and let N, L ∈ U . By Theorem 3.6.9, (N Mr )r∈Z and (L Mr )r∈Z are excellent filtrations of the σ-∗ -R-modules N and L, respectively, so one can consider the corresponding σ-∗ -dimension polynomials ΦN (t) and ΦL (t) associated with these filtrations. It is easy to see that the inclusion N ⊆ L implies the inequality ΦN (t) ≤ ΦL (t) (where ≤ denotes the natural order on the set of numerical polynomials in one variable: f (t) ≤ g(t) if and only if f (r) ≤ g(r) for all sufficiently large r ∈ Z). Furthermore, L = N if and only if ΦN (t) = ΦL (t). (Indeed, if ΦN (t) = ΦL (t), but L
N , then there exists r0 ∈ Z such that  L Mr
N Mr for all r > r0 . In this case one would have ΦL (t) < ΦN (t) that contradicts our assumption.) Let us prove that if L, N ∈ BU = {(P, Q) ∈ U ×U |P ⊇ Q} and µU (N, L) ≥ d (d ∈ Z, d ≥ −1), then deg (ΦN (t) − ΦL (t)) ≥ d. We proceed by induction on d. Since deg (ΦN (t) − ΦL (t)) ≥ −1 for any pair (P, Q) ∈ BU and deg (ΦN (t) − ΦL (t)) ≥ 0 if N  L (as we have noticed, in this case ΦN (t) > ΦL (t)), our statement is true for d = −1 and d = 0. Now, let d > 0 and let for any i ∈ N, i < d, the inequality µU (N, L) ≥ i ( (N, L) ∈ BU ) imply that deg (ΦN (t) − ΦL (t)) ≥ i. Suppose that (N, L) ∈ BU and µU (N, L) ≥ d. Then there exists an infinite strictly descending chain N = N 0  N1  · · ·  L

CHAPTER 3. DIFFERENCE MODULES

242

∗ of σ-  M such that µU (Ni−1 , Ni ) ≥ d − 1 for i = 1, 2, . . . . If  -R-submodules of deg ΦNi−1 (t) − ΦNi (t) ≥ d for some i ≥ 1, then deg (ΦN (t) − ΦL (t)) ≥ d and our statement is proved.   Suppose that deg ΦNi−1 (t) − ΦNi (t) = d − 1 for all i = 1, 2, . . . . Then every polynomial ΦNi−1 (t) − ΦNi (t) (i ∈ N, i ≥ 1) can be written in the 

d−1  t+k where ci0 , . . . , ci,d−1 ∈ Z, ci,d−1 > 0 form ΦNi−1 (t) − ΦNi (t) = cik k k=0 

d−1  t+k  where cik (see Theorem 3.6.4). In this case, ΦN (t) − ΦNi (t) = k

ci0 , . . . , ci,d−1 ∈ Z and ci,d−1 =

i 

k=0

cν,d−1 . Therefore, c1,d−1 < c2,d−1 < . . .

ν=1

whence deg (ΦN (t) − ΦL (t)) ≥ d for any pair (N, L) ∈ B with µU (N, L) ≥ d. The last statement, together with the fact that deg (ΦN (t) − ΦL (t)) ≤ m+n for any (N, L) ∈ B, implies that µU (N, L) ≤ m + n for all pairs (N, L) ∈ B. It follows that (3.6.5) typeU M ≤ m + n . If σ∗ dimR M = q > 0, then Theorem 3.6.14 shows that M contains q elements z1 , . . . , zq that are σ-∗ -linearly independent over the ring of σ-∗ operators F. Clearly, if U  is the family of all F -submodules of the left F -module Fz1 , then typeU
Fz1 ≤ typeU M . Thus, in order to prove the first statement of the theorem, it suffices to show that typeU
Fz1 ≥ m + n. Let U  denote the family of all F-submodules of Fz1 that can be represented as finite km (β1 − 1)l1 . . . (βn − 1)ln z1 where sums of F-modules of the form Fα1k1 . . . αm k1 , . . . , km , l1 , . . . , ln ∈ N. We are going to use induction on m + n to show that µU

(Fz1 , 0) ≥ m + n. If m + n = 0, inequality is trivial. Let m + n > 0 that is at least one of the numbers m, n is positive. Suppose, first, that m > 0. Since the element z1 is linearly independent over F, we have the strictly descending chain 2 z1  · · ·  0 Fz1  Fαm z1  Fαm i−1 i of elements of U  . Let us show that µU

(Fαm z1 , Fαm z1 ) ≥ m + n − 1 for all i−1 i i = 1, 2, . . . . Let L = Fαm z1 /Fαm z1 and let y be the image of the element i−1 i−1 z1 under the natural epimorphism Fαm z1 → L. Then αm y = y and αm αm θ y = θ αm y for any element θ ∈ Θ , where n m−1   Θ = { ανuν (βµ − 1)vµ |u1 , . . . , um−1 , v1 , . . . , vn ∈ N}. ν=1 µ=1 

Setting σ = {α1 , . . . , αm−1 }, treating R as a σ  -∗ -field, and denoting the corresponding ring of σ  -∗ -operators by F  , we obtain that L = F  y and the elei−1 i z1 , Fαm z1 ) = ment y is linearly independent over F  . Furthermore, µU

(Fαm   µU1

(L, 0) where U1 is the family of all F -submodules of L that can be represented as finite sums of the F  -modules of the form F  θ y (θ ∈ Θ ). By the induci−1 i z1 , Fαm z1 ) ≥ m+n−1 tion hypothesis, µU1

(L, 0) ≥ m+n−1 whence µU

(Fαm (i = 1, 2, . . . ). Thus, µU

(Fz1 , 0) ≥ m + n.

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243

Now, let m + n > 0 and n > 0. Let us consider the strictly descending chain of F-modules Fz1  F(βn − 1)z1  F(βn − 1)2 z1  · · ·  0. As in the case m > 0, for every i = 1, 2, . . . , one can consider the σ-∗1 -R-module F(βn − 1)i−1 z1 /F(βn − 1)i z1 (where 1 = {β1 , . . . , βn−1 }) and obtain that µU

(F(βn − 1)i−1 z1 , F(βn − 1)i z1 ) ≥ m + n − 1 whence µU

(Fz1 , 0) ≥ m + n. Thus, typeU M ≥ typeU
Fz1 ≥ typeU

Fz1 ≥ m + n. Combining these inequalities with (3.6.5), we obtain that typeU M = m + n. Our proof of the last equality shows that for every ν = 1, . . . , q, typeUν
Fzν = m + n where U!ν denotes the family of"all F-submodules of the F-module F zν . k k−1 = m + n = typeU M for k = 1, . . . , q Therefore, µU ν=1 Fzν , ν=1 Fzν (if k = 0, the sum is 0), so the chain M=

q 

Fzν 

ν=1

shows that

q−1 

Fzν  · · ·  Fz1  0

ν=1

dimU M ≥ σ∗ dimR M .

(3.6.6)

Let N0  N1  · · ·  Np be a chain of σ-∗ -R-submodules of M such that µU (Nk−1 , Nk ) = typeU M = m + n for k = 1, . . . , p. The arguments used at the beginning of the proof show that the degree of each polynomial ΦNk−1 (t)−ΦNk (t) (1 ≤ k ≤ p) is equal to m + n, so that such a polynomial can be written as ΦNk−1 (t) − ΦNk (t) =

m−1 

  

n t+i t+m+j + 2j bkj i m+j j=0

aki

i=0

where aki , bkj ∈ Z(0 ≤ i ≤ m − 1, 0 ≤ j ≤ n) and bkn = σ∗ dimR Ni−1 − σ∗ dimR Ni ≥ 1. It follows that ΦN0 (t) − ΦNp (t) =

p   i=1

    n 
t+i   m−1 t+m+j + ΦNi−1 (t) − ΦNi (t) = ai 2j b
j i m+j i=0 j=0

where ai , bj ∈ Z(0 ≤ i ≤ m − 1, 0 ≤ j ≤ n) and bn = On the other hand, ΦN0 (t) − ΦNp (t) ≤ ΦN0 (t) ≤ ΦM (t) =

m−1  i=0

i=1 bkn

≥ p.

  

n t+i t+m+j + 2j bj i m+j j=0

ai

p

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where bn = σ∗ dimR M. (We write f (t) ≤ g(t) for two numerical polynomials f (t) and g(t) if f (r) ≤ g(r) for all sufficiently large r ∈ Z.) It follows that bn ≥ bn ≥ p whence σ∗ dimR M ≥ dimU M . Combining the last inequality with (3.6.6) we obtain the desired equality dimU M = σ∗ dimR M . Now, let us prove the last statement of the theorem. If σ∗ dimR M = 0, then for any pair (N, L) ∈ BU , we have the equality σ∗ dimR N = σ∗ dimR L = 0 whence deg ΦN (t) < m+n, deg ΦL (t) < m+n, and deg (ΦN (t) − ΦL (t)) < m+n. As it has been shown, the last inequality implies the inequality µU (N, L) <  m + n. Thus, if σ∗ dimR M = 0, then typeU M < m + n. Exercise 3.6.25 Let K be a G-field, F the ring of G-operators over K, M a finitely generated G-K-module, and U the set of all F-submodules of M . Prove the following statements: (a) If δG(M ) > 0, then typeU M = rank G and dimU M = δG(M ). (b) If δG(M ) = 0, then typeU M ≤ tG(M ).

Chapter 4

Difference Field Extensions 4.1

Transformal Dependence. Difference Transcendental Bases and Difference Transcendental Degree

Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let T be the free commutative semigroup generated by σ (this semigroup was introduced in Section 2.1). If L is a difference (σ-) field extension of K and A ⊆ L, then an element v ∈ L is said to be transformally dependent or σ-algebraically dependent on a set A over K if v is σ-algebraic over the field KA. Otherwise, v is said to be transformally (or σ-algebraically) independent on A over K. The fact that an element v ∈ L is σ-algebraically dependent on a set A over K will be written as v ≺K A (if this is not the case, we write v ⊀K A). Exercises 4.1.1 Let K be a difference field with a basic set σ and L a σ-field extension of K. 1. Prove that an element v ∈ L is σ-algebraically dependent on a set A ⊆ L if and only if there exists a finite family {η1 , . . . , ηs } ⊆ A such that v is σ-algebraic over Kη1 , . . . , ηs . 2. Prove that a set S ⊆ L is σ-algebraically dependent over K if and only if there exists an element s ∈ S such that s ≺K S \ {s}. Theorem 4.1.2 Let K be a difference field with a basic set σ = {α1 , . . . , αn }, let L be a σ-field extension of K and let η and ζ be two elements in L. Then (i) If ζ is σ-algebraic over Kη and η is σ-algebraic over K, then ζ is σ-algebraic over K. (ii) If ζ is σ-algebraic over Kη and η is σ-transcendental over Kζ, then ζ is σ-algebraic over K. (iii) The set of all elements of L that are σ-algebraic over K is an intermediate σ-field of L/K. 245

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(iv) If X ⊆ L and every element of X is σ-algebraic over K, then KX is a σ-algebraic extension of K. (v) Let M be a σ-field extension of L. Then M is σ-algebraic over K if and only if M is σ-algebraic over L and L is σ-algebraic over K. (vi) The relation ≺K is the dependence relation from L to the power set of L. In other words, ≺K satisfies conditions (i) - (iv) of Definition 1.1.4. PROOF. (i) Let us consider a ring of σ-polynomials in one σ-indeterminate y over L and let us fix an orderly ranking < of the set of terms T y = {τ y | τ ∈ T }. (The concepts of ranking and orderly ranking were introduced at the beginning of Section 2.4). Since η is σ-algebraic over K, there exists a σ-polynomial f ∈ K{y} such that f (ζ) = 0. Let u = τ1 y be the leader of f and let r1 = ord τ1 . Then τ1 η ∈ K({τ η | τ y < τ1 y}) hence τ  τ1 η ∈ K({τ η | τ y < τ  τ1 y}) for every τ  ∈ T . It follows that for any r ≥ r1 , every element of the set T (r)η belongs to the field K((T (r) \ T (r − r1 ))η), so that K((T (r)η) = K( (T (r) \ T (r − r1 ))τ1 η).

(4.1.1)

(As in Section 3.1, for any r ∈ N, T (r)denotes the set of all power products n τ = α1k1 . . . αnkn ∈ T such that ord τ = i=1 ki ≤ r}; also, for any set T  ⊆ T and for any ξ ∈ L, T  ξ denotes the set {τ (ξ)|τ ∈ T  }.) Similarly, the fact that ζ is σ-algebraic over Kη implies that there exists τ2 ∈ T such that τ2 ζ ∈ Kη({θζ | θy < τ2 y}). It follows that there exists q ∈ N such that τ2 ζ ∈ K(T (q)η ∪ {θζ | θy < τ2 y}), and therefore τ  τ2 ζ ∈ K(T (q + ord τ  )η ∪ {θζ | θy < τ  τ2 y}) for every τ  ∈ T . Let r2 = ord τ2 . Then the last inclusion implies that for any s ≥ r2 K(T (s)ζ) ⊆ K((T (s + q)η ∪ (T (s) \ T (s − r2 )τ2 )ζ). Taking into account (4.1.1) we obtain that K(T (s)ζ) ⊆ K((T (s + q) \ T (s + q − r1 )τ1 )η ∪ (T (s) \ T (s − r2 )τ2 )ζ) (4.1.2) for any s > max{r1 − q, r2 }. By formula (1.4.16),  thennumber of generators τ ζ in s s+n = the left-hand side of (4.1.2) is equal to + o(sn ), while the number n n! of  is given by  in

the right-hand  the alternating sum  side

of (4.1.2)

generators s − r2 + n s + q + n − r1 s+n s+q+n − − , which is a numerical + n n n n polynomial in s of degree less than n. Applying Proposition 1.6.28(iii), we obtain that for sufficiently large s, the set T (s)ζ is algebraically dependent over K hence ζ is σ-algebraic over K. (ii) If ζ is σ-algebraic over Kη, then there exists τ0 ∈ T such that τ0 η is algebraic over the field N = Kη({τ ζ | τ y < τ0 y}). It follows that there is a polynomial f (X) in one variable X with coefficients in N such that f (τ0 ζ) = 0.

4.1. DIFFERENCE TRANSCENDENTAL BASES

247

If some coefficient of f contains a transform τ η, then the last equality would imply that η is σ-algebraic over Kζ. Since this is not true, all coefficients of the polynomial f (X) belong to K({τ ζ | τ y < τ0 y}). Therefore, τ0 ζ is algebraic over the last field, hence ζ is σ-algebraic over K. (iii) Let L0 = {u ∈ L | u is σ-algebraic over K}. Let η, ζ ∈ L0 and let ξ denote one of the elements η + ζ, ηζ, ηζ −1 (if ζ = 0), or αi η (1 ≤ i ≤ n). Since ξ is σ-algebraic over Kη, ζ = Kηζ, ζ is σ-algebraic over Kη and η is σ-algebraic over K, part (i) implies that ξ is σ-algebraic over K, that is, ξ ∈ L0 . (iv) If X ⊆ L and every element of X is σ-algebraic over K, then part (iii) shows that KX ⊆ L0 , so that KX is a σ-algebraic extension of K. (v) Clearly, if K ⊆ L ⊆ M is a sequence of σ-field extensions and M/K is σ-algebraic, then M/L and L/K are also σ-algebraic. Conversely, suppose that L is a σ-algebraic extension of K and M is a σ-algebraic extension of L. If u ∈ M , then there exist finitely many elements v1 , . . . , vk ∈ L such that u is σ-algebraic over Kv1 , . . . , vk . Since for every i = 2, . . . , k, vi is σ-algebraic over Kv1 , . . . , vi−1 , one can apply part (i) and obtain that u is σ-algebraic over K. (vi) The fact that the relation ≺K satisfies the first two conditions of Definition 1.1.4 is obvious. Let S, U and V be three subsets of L such that S ≺K U and U ≺K V , that is, every element of S is σ-algebraic over KU  and every element of U is σ-algebraic over KV . Applying parts (iv)
and
(v) to the sequence of σ-field extensions KV  ⊆ KV ∪ U  ⊆ KV U S we obtain

that the extension KV U S/KV  is σ-algebraic. Therefore every element of S is σ-algebraic over KV , so the relation ≺K satisfies condition (iii) of Definition 1.1.4. In order to prove that ≺K satisfies the last condition of the definition as well, suppose that a set S ⊆ L and elements s ∈ S, u ∈ L have the property that u ≺K S, but u ⊀K S \ {s}. Then u is σ-algebraic over KS and σ-transcendent over KS \ {s}. It follows that there exists a σ-polynomial f in one σ-indeterminate y with coefficients in KS such that f (u) = 0. Since some of the coefficients of f contain s (otherwise, u would be σ-algebraic
over KS\{s}), the last equality implies that s is σ-algebraic over K(S\{s}) {u}. This completes the proof of the theorem.  The last part of Theorem 4.1.2 and Proposition 1.1.6 imply the following properties of σ-algebraic dependence. We leave the proof of the corresponding statements to the reader as an exercise. Corollary 4.1.3 Let K be a difference field with a basic set σ, L a σ-field extension of K, S ⊆ L and u ∈ L. Then independent over K and u is σ-transcendental (i) If the set S is σ-algebraically
over KS, then the set S {u} is σ-algebraically independent over K. (ii) Suppose that u ≺K S and S ≺K V for some set V ⊆ L (that is, every element of S is σ-algebraic over KV ). Then u ≺K V (iii) Let v1 , . . . , vm ∈ L (m ≥ 2) and let u ≺K {v1 , . . . , vm }, but u ⊀K {v1 , . . . , vm−1 }. Then vm ≺K {u, v1 , . . . , vm−1 }

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(iv) Let S  ⊆ S and let a finite set {s1 , . . . , sk } of elements of S be σ-algebraically independent over K. Furthermore, suppose that si ≺K S  for i = 1, . . . , k. Then there exist elements u1 , . . . , uk ∈ S  such that ui ≺K (S  \ {u1 , . . . , uk }) ∪  {s1 , . . . , sk } (1 ≤ i ≤ k). Definition 4.1.4 Let K be a difference field with a basic set σ, L a σ-field extension of K, and A ⊆ L. A set B ⊆ A is called a basis for transformal transcendence or a difference (or σ-) transcendence basis of A over K if B is a maximal σ-algebraically independent over K subset of A. In other words, a set B ⊆ A is a σ-transcendence basis of A over K if B is σ-algebraically independent over K and any subset of A containing B is σ-algebraically dependent over K. If A = L, the set B is called a basis for transformal transcendence or a difference (or σ-) transcendence basis of L over K. (In this case we also say that B is a σ-transcendence basis of the extension L/K.) By Zorn’s Lemma (see Proposition 1.1.3(v)), every subset of the σ-field L (we use the notation of the last definition) has a σ-transcendence basis over K. Furthermore, the last statement of Theorem 4.1.2 implies the following two consequences of Proposition 1.1.8. Proposition 4.1.5 Let K be a difference field with a basic set σ, L a σ-field extension of K, and A ⊆ L. Then the following conditions on a set B ⊆ A are equivalent. (i) B is a σ-transcendence basis of A over K. (ii) The set B is σ-algebraically independent over K and every element of A is σ-algebraic over KB. (iii) B is a minimal subset of A with respect to the property A ≺K B. (In other words, every element of A is σ-algebraic over KB, but not over KB0   if B0
B.) Proposition 4.1.6 Let K be a difference field with a basic set σ and L a σ-field extension of K. Then any two σ-transcendence bases of L over K have the same cardinality.  Definition 4.1.7 Let K be a difference field with a basic set σ, L a σ-field extension of K, and A ⊆ L. Then the difference (or σ-) transcendence degree of A over K is the number of elements of any σ-transcendence basis of A over K, if this number is finite, or infinity in the contrary case. With the notation of Definition 4.1.7, the σ-transcendence degree of A over K is denoted by σ-trdegK A (in particular, if A does not have finite σ-transcendence bases over K, we write σ-trdegK A = ∞). If A = L, then the σ-transcendence degree of L over K is also called a σ-transcendence degree of the extension L/K. A difference (σ-) field extension L/K is said to be purely σ-transcendental if there is a σ-algebraically independent over K set B ⊆ L such that L = KB.

4.1. DIFFERENCE TRANSCENDENTAL BASES

249

Theorem 4.1.8 Let K be a difference field with a basic set σ and L a σ-field extension of K. (i) Any family of σ-generators of L over K contains a σ-transcendence basis of this difference field extension. If the σ-field K is inversive and L a σ ∗ -overfield of K, then any system of σ ∗ -generators of L over K contains a σ-transcendence basis of L over K. (ii) If a set A ⊆ L is σ-algebraically independent over K, then it can be extended to a σ-transcendence basis B ⊇ A of L/K. (iii) Let η1 , . . . , ηm ∈ L. Then σ-trdegK Kη1 , . . . , ηm  ≤ m. If K is inversive and L a σ ∗ -overfield of K, then σ-trdegK Kη1 , . . . , ηm ∗ ≤ m. (iii) Let {η1 , . . . , ηm } and {ζ1 , . . . , ζs } be two finite subsets of L such that Kη1 , . . . , ηm  = Kζ1 , . . . , ζs  (or Kη1 , . . . , ηm ∗ = Kζ1 , . . . , ζs ∗ if K is inversive and L a σ ∗ -overfield of K). If the set {ζ1 , . . . , ζs } is σ-algebraically independent over K, then s ≤ m. (iv) Let S = Ky1 , . . . , ys } be the ring of difference (σ-) polynomials in σindeterminates y1 , . . . , ys over K. If k = s, then S cannot be a ring of σpolynomials in k σ-indeterminates over K. PROOF. Statement (ii) and the fact that every family of σ-generators of L over K contains a σ-transcendence basis of L/K are direct consequences of Proposition 1.1.8(iii). If the σ-field K is inversive and L = KS∗ , then the first part of statement (i) shows that S contains some σ-transcendence basis B of the σ-field extension KS/K. It is easy to see that B is also a σ-transcendence basis of L/K. Indeed, if a ∈ L, then there exists τ ∈ T such that τ (a) ∈ KS. It follows that τ (a) is σ-algebraic over KB hence a is also σ-algebraic over the last σ-field. The first part of statement (iii) follows from (i), since the system of σgenerators {η1 , . . . , ηm } of Kη1 , . . . , ηm /K contains a σ-transcendence basis of this extension which consists of σ-trdegK Kη1 , . . . , ηm  elements. Similarly, the second part of (iii) immediately follows from (ii). The last statement of the theorem is a direct consequence of Proposition 4.1.6.  The following theorem shows that the difference transcendence degree is additive over a tower of fields. Theorem 4.1.9 Let K be a difference field with a basic set σ, L a σ-field extension of K, and M a σ-field extension of L. Let A be a σ-transcendence basis of the extension L/K and B a σ-transcendence basis of M/L. Then A ∪ B is a σ-transcendence basis of M/K and therefore, σ-trdegK M = σ-trdegL M + σ-trdegK L. PROOF. Since the set B is σ-algebraically independent over L, it is σ
algebraically independent over KA. Therefore, the set A B is σ-algebraically independent over K. To complete the proof one has to show that if u ∈ M , then
u is σ-algebraic over KA B. Since B is a σ-transcendence basis of M/L,

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there exists a non-zero difference (σ-) polynomial f (y) in one σ-indeterminate y with coefficients in LB such that f
(u) = 0. If S denotes the set of coefficients of f , then u
is σ-algebraic over KS B, so that u is σ-algebraically dependent
S B). Since S ≺K A, on the set S
B over K
(as before, we write this as u ≺ K
we have S B ≺K A B whence u ≺K A B (see Corollary 4.1.3(ii)). This completes the proof.  The following statement is an analogue of Proposition 1.6.31 for difference field extensions. We leave its proof to the reader as an exercise. Proposition 4.1.10 Let K be a difference field with a basic set σ and N a σ-field extension of K. (i) If F and G are two intermediate σ-fields of N/K, then their compositum F G is a σ-subfield of N and σ-trdegF F G ≤ σ-trdegK G. Furthermore, σ-trdegK F G ≤ σ-trdegK F + σ-trdegK G. (ii) Let K ⊆ L ⊆ M ⊆ N be a sequence of difference (σ-) field extensions and A ⊆ M \ L. Then σ-trdegK A LA ≤ σ-trdegK L. If either A is σ-algebraically independent over L or every element of A is σ-algebraic over K, the last inequality becomes an equality.  Exercises 4.1.11 1. Find an example of a purely σ-transcendental difference (σ-) field extension L/K with two σ-transcendental bases B and C such that L = LB but LC is a proper σ-subfield of L. 2. Let K be a difference field with a basic set σ, M a σ-field extension of K and L an intermediate σ-field of M/K. Furthermore, let S be a σ-algebraically independent over L subset of M . Prove that the σ-field extension LS/KS is σ-algebraic if and only if L/K is σ-algebraic. We conclude this section with an alternative description of the difference transcendence degree of a finitely generated difference field extension. This description is due to P. Evanovich [61]. Let K be a difference field with a basis set σ = {α1 , . . . , αn } and let σk = {α1 , . . . , αk } for k = 1, . . . , n − 1. If S is any set in a σ-overfield of K and σ  ⊆ σ, then KSσ
will denote the σ  -overfield of K generated by S. (If σ  = {αi1 , . . . , αik }, then KSσ
coincides with the field K({αid11 . . . αidkk (a) | a ∈ S, d1 , . . . , dk ∈ N}).) In the case σ  = σ we write KS instead of KSσ . As usual, if Σ is a subset of a ring L and φ an endomorphism of L, then φ(Σ) denotes the set {φ(a) | a ∈ Σ}. Furthermore, if α is an endomorphism and Φ is a set of endomorphisms of L , then αΦ = {αφ | φ ∈ Φ} and Φ(Σ) = {φ(a) | φ ∈ Φ, a ∈ Σ}. Let L = KS be a σ-field extension of K generated by a finite set S
k and for every k = 1, 2, . . . , let Lk = K j=0 αnj (S)σn−1 and δk = σn−1 trdegLk−1 Lk . (A). Applying Proposition 4.1.10, one can easily see that δk = σn−1 -trdegαn (Lk−1 ) αn (Lk ) ≥ σn−1 -trdegLk Lk+1 for k = 1, 2, . . . . (The fields Lk and Lk+1 are obtained from the fields αn (Lk−1 ) and αn (Lk ), respectively, by adjoining (in the sense of σn−1 -field extensions) the same set [K \ αn (K)] ∪ S.)

4.1. DIFFERENCE TRANSCENDENTAL BASES

251

Theorem 4.1.12 With the above notation, σ-trdegK L = min{δk | k = 1.2, . . . } (= lim δk ). k→∞

PROOF. It follows from the definition of the non-increasing sequence {δk } that there exists k0 ∈ N such that δk = min{δk | k = 1.2, . . . } for all k ≥ k0 . Since αnk0 (S) is a set of σn−1 -generators of the σn−1 -field extension Lk0 /Lk0 −1 , we can choose a set B ⊆ S such that αnk0 (B) is a σn−1 -transcendence basis of Lk0 /Lk0 −1 . Then for every p ∈ N, αnk0 +p (B) is a σn−1 -transcendence basis of αnp (Lk0 )/αnp (Lk0 −1 ) and so must contain a σn−1 -transcendence basis of Lk0 +p /Lk0 +p−1 . Since δk0 = δk0 +p , we obtain that αnk0 +p (B) itself is a σn−1 -transcendence basis of Lk0 +p /Lk0 +p−1 . Denoting the free commutain−1 (i1 , . . . , in−1 ∈ N) by Tσn−1 , tive semigroup of all power products α1i1 . . . αn−1 ∞  one can easily observe that the set αnk0 +p Tσn−1 (B) is algebraically indep=0

pendent over K, so that αnk0 (B) is σ-algebraically independent over K. Since αnk0 (B) can be extended to a σ-transcendence basis of L/K, we obtain that min{δk | k = 1.2, . . . } = Card αnk0 (B) ≤ σ-trdegK L. (As it will follow from the rest of the proof, αnk0 (B) is a σ-transcendence basis of L/K.) To prove the opposite inequality, let ∆ = σ-trdegK L and let c = σn−1 trdegK Lk0 −1 . Then for any p ∈ N, σn−1 -trdegK Lk0 +p = (p + 1)δk0 + c. If Z is a σ-transcendence basis of L/K contained in S, then, for every p ∈ k 0 +p αni (Z) is σn−1 -algebraically independent over K. Since this set N, the set i=0

contains (k0 + p + 1)∆ elements, (p + 1)δk0 + c ≥ (k0 + p + 1)∆. Letting p → ∞  we see that δk0 ≥ ∆ whence δk0 = σ-trdegK L. Corollary 4.1.13 Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let L = KS be a σ-field extension of K generated by a finite set S. As k  αnj (S)σn−1 and δk = σn−1 -trdegLk−1 Lk for k = 1, 2, . . . before, let Lk = K j=0

(as we have seen, δ1 ≥ δ2 ≥ . . . ). Then there exists a finite set Z ⊆ S such that Z is a σ-transcendence basis of L/K and if k0 = min{δk | k = 1, 2, . . . } (so that σn−1 -trdegLk−1 Lk = σ-trdegK L for all k ≥ k0 ), then αnk (Z) is a σn−1 transcendence basis of Lk over Lk−1 for k = k0 , k0 + 1, . . . . PROOF. It follows from the prof of Theorem 4.1.12 that if we choose Z ⊆ S such that αnk (Z) is a σn−1 -transcendence basis of Lk /Lk−1 for all k ≥ k0 , then αnk0 (Z) is a σn−1 -transcendence basis of L/K. Clearly, Card Z = Card αnk0 (Z) = σ-trdegK L and any relation of σ-algebraic dependence of elements of Z over K yields a relation of σ-algebraic dependence of the corresponding elements of  αnk0 (Z) over K. Therefore, Z is a σ-transcendence basis of L over K. Let K be a difference field with a basic set σ = {α1 , . . . , αn }. In what follows, for any set σ  = {αi1 , . . . , αiq } ⊆ σ, Tσ
will denote the free commutative

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

252

semigroup generated by σ  . Furthermore, let σj = {α1 , . . . , αj } (0 ≤ j ≤ n with σ0 = ∅, σn = σ) and for any set S ⊆ K and any integers in , . . . , it ∈ N (1 ≤ t ≤ n), let ∗

S (in , . . . , it ) =

in −1 

αnj Tσn−1 (S)



j=0 i t −1

in−1 −1



j αnin αn−1 Tσn−2 (S)



...

j=0

t+1 j αnin . . . αt+1 αt Tσt−1 (S) and S(in , . . . , it ) = S ∗ (in , . . . , it + 1).

i

j=0

Let L be a σ-field extension of K generated by a finite set S. It follows from the proof of Theorem 4.1.12. that one can choose kn ∈ N such that σtrdegK L = σn−1 -trdegK S(in )σn−1 KS ∗ (in )σn−1 for any in ≥ kn . Using the same argument we obtain that there exists kn−1 ∈ N such that σ-trdegK L = σn−2 -trdegK S(kn ,in−1 )σn−2 KS ∗ (kn , in−1 )σn−2 for any in−1 ≥ kn−1 . Since the endomorphism αn is injective and both fields KS(in , in−1 )σn−2 and KS ∗ (in , in−1 )σn−2 can be obtained by adjoining the same set to the σn−2 -fields αnin −kn (KS(kn , in−1 )σn−2 ) and αnin −kn (KS ∗ (kn , in−1 )σn−2 ), respectively, we have σ-trdegK L =

σn−1 -trdegK S(in )σn−1 KS ∗ (in )σn−1

≤

σn−2 -trdegK S ∗ (in ,in−1 )σn−2 KS(in , in−1 )σn−2

≤

σn−2 -trdegαinn −kn (K S ∗ (in ,in−1 )σ

≤

×(KS(in , in−1 )σn−2 ) σ-trdegK L.

n−2

in −kn ) αn

Thus, if in ≥ kn and in−1 ≥ kn−1 , then σ-trdegK L = σn−2 -trdegK S ∗ (in ,in−1 )σn−2 KS(in , in−1 )σn−2 . k

n−1 (Z) is a σn−2 -transcendence Furthermore, if Z ⊆ S is chosen so that αnkn αn−1 in−1 ∗ (Z) is a basis of KS(kn , kn−1 )σn−2 over KS (kn , kn−1 )σn−2 , then αnin αn−1 σn−2 -transcendence basis of KS(in , in−1 )σn−2 over KS ∗ (in , in−1 )σn−2 for any in ≥ kn and in−1 ≥ kn−1 . Corollary 4.1.13 shows that αnin (Z) is a σn−1 transcendence basis of KS(in )σn−1 over KS ∗ (in )σn−1 and thus, Z is a σtranscendence basis of L over K. Extending these arguments by induction we obtain that there exist positive integers k1 , . . . , kn and a set Z ⊆ S such that (i) Z is a σ-transcendence basis of L/K; (ii) if t ∈ {1, . . . , n} and iν ≥ kν for ν = t, . . . , n, then αnin . . . αtit (Z) is a σt−1 -transcendence basis of KS(in , . . . , it )σt−1 over KS ∗ (in , . . . , it )σt−1 . (As above, σt−1 = {α1 , . . . , αt−1 }.) In particular,

σt−1 -trdegK S ∗ (in ,...,it )σt−1 KS(in , . . . , it )σt−1 = σ-trdegK L.

4.1. DIFFERENCE TRANSCENDENTAL BASES

253

Definition 4.1.14 With the above notation, a σ-transcendence basis Z of L/K satisfying conditions (i) and (ii) is called a limit σ-transcendence basis of L over K (or a limit σ-transcendence basis of the σ-field extension L/K). The following example shows that not every σ-transcendence basis is a limit one. Example 4.1.15 Let us consider the field of complex numbers C as an ordinary difference field with a basic set σ = {α} where α is the identity automorphism of C. Let X = {x0 , x1 , . . . } and Y = {y0 , y1 , . . . } be two
denumerable sets of elements in some overfield of C such that the set X Y is algebraically independent over C. Furthermore, let
D = {d1 , d2 , . . . } be the set of elements in the algebraic closure of the field C(X Y ) such that d2j = xj + yj (j = 1, 2, . . . )
and let L = C(X Y ∪D). Then the field L can be treated as extension
a σ-field
of C if we define the action of α on the set of generators X Y D by α(xj ) = xj+1 , α(yj ) = yj+1 , and α(dj+1 ) = dj+2 (j = 0, 1, . . . ). Clearly, L = CS, where S = {x0 , y0 , d1 }, and Z = {x0 , y0 } ⊆ S is a σ-transcendence basis of L/C. However, Z is not a limit σ-transcendence basis of L over C. Indeed, for any k k−1   αi (S)), since k ≥ 1, αk (Z) is not a transcendence basis of C( αi (S))/C( i=0

i=0

the set {α(x0 ), α(y0 )} is algebraically dependent over C(x0 , y0 , d1 ) (α(x0 ) + α(y0 ) = d21 ). Exercise 4.1.16 Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let L = KS be a σ-field extension of K generated by a finite set S. Let Z be a limit σ-transcendence basis of L/K that satisfy condition (ii) (stated before Definition 4.1.14) for some positive integers k1 , . . . , kn . Prove that if i1 , . . . , in ∈ N and iν ≥ kν (ν = 1, . . . , n), then the set Tσ (Z) \ Z(i1 , . . . , in ) is algebraically independent over K(S(i1 , . . . , in )). In the formulation of the next theorem we use the following convention. Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let K{y1 , . . . , ys } be a ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K. By the order of a term u = α1k1 . . . αnkn yj (1 ≤ j ≤ s) with respect to αi (1 ≤ i ≤ n) we mean the integer ki ; it will be denoted by ordαi u. If f is a σ-polynomial in K{y1 , . . . , ys } and u1 , . . . , um are all terms that appear in f , then by the order of f with respect to αi (denoted by ordαi f ) we mean max{ordαi uν |1 ≤ ν ≤ m}. Theorem 4.1.17 Let K be a difference field with a basic set σ, α ∈ σ, and σα = σ \ {α}. Let K{y} be the ring of σ-polynomials in one σ-indeterminate y over K and let L be a σ-overfield of K. Furthermore, let an element η ∈ L be σ-algebraic over K and let r ∈ N be the smallest integer such that there is a σ-polynomial A ∈ K{y} that vanishes at η and whose order with respect to α is r. Then σα -trdegK Kη = r. PROOF. By the condition of the theorem, elements η, . . . , αr−1 (η) are σα -algebraically independent over K and αr (η) is σα -algebraic over the field

254

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

Kη, . . . , αr−1 (η)σα . (In this case we treat K as a difference field with the basic set σα and consider its σα -field extension.) Let us show that every element αk (η), k ≥ r, is σα -algebraic over Kη, . . . , αk−1 (η)σα . We proceed by induction on k. As we have seen, our statement is true for k = r. Suppose it is true for some k > r, so that αk (η) is σα -algebraic over Kη, . . . , αk−1 (η)σα . Then αk+1 (η) is σα -algebraic over the σα -field α(K)α(η), . . . , αk (η)σα and hence over its overfield Kη, . . . , αk (η)σα . It follows (see Theorem 4.1.2(v)) that every element αk (η) k ≥ r, is σα algebraic over Kη, . . . , αr−1 (η)σα , so that the field extension  Kη/Kη, . . . , αr−1 (η)σα is σα -algebraic, whence σα -trdegK Kη = r. We conclude this section with some some results on ordinary difference field extensions. The following statement is a direct consequence of Theorem 4.1.17. Corollary 4.1.18 Let K be an ordinary difference field with a basic set σ = {α}, K{y} the ring of σ-polynomials in one σ-indeterminate y over K, and L an overfield of K. Furthermore, let an element η ∈ L be σ-algebraic over K and let r ∈ N be the smallest integer such that there is a σ-polynomial A ∈ K{y} of  order r that vanishes at η. Then trdegK Kη = r. Let K be an ordinary difference field with a basic set σ = {α} and let L = KS be a σ-field extension of K generated by a set S. Then one has the descending chain of σ-fields L = KS ⊇ Kα(S) ⊇ Kα2 (S) ⊇ . . . (as usual, for any automorphism τ of L and any set Σ ⊆ L, τ (L) denotes the set {τ (a) | a ∈ S}). Suppose that trdegK L < ∞ (hence σ-trdegK L = 0) and let ri = trdegK Kαi (S) (i = 0, 1, 2, . . . ). Then r0 = trdegK L ≥ r1 ≥ r2 ≥ . . . , hence there exists a finite limit s = limi→∞ ri = min{ri | i ∈ N} and there exists k ∈ N such that s = rk . Proposition 4.1.19 With the above notation and conventions, s = trdegK ∗ L∗ where ∗ stands for inversive closure. PROOF. Let elements η1 , . . . , ηp ∈ L∗ be algebraically independent over K . Then there exists m ∈ N such that αm (η1 ), . . . , αm (ηp ) lie in L. Then αk+m (η1 ), . . . , αk+m (ηp ) form a subset of Kαk (S) which is algebraically independent over K ∗ and hence over K. It follows that s = trdegK Kαk (S) ≥ p. Therefore, s ≥ trdegK ∗ L∗ . To prove the opposite inequality, let us choose a transcendence basis {ζ1 , . . . , ζs } of Kαk (S) over K and show that the elements of this basis are algebraically independent over K ∗ . Suppose that ζ1 , . . . , ζs are algebraically dependent over K ∗ . Then there is a finite set Σ ⊂ K ∗ such that ζ1 , . . . , ζs are algebraically dependent over KΣ. Furthermore, one can choose d ∈ N such that αd (Σ) ⊆ K. Now it is easy to see that αd (ζ1 ), . . . , αd (ζs ) are algebraically dependent over αd (K)αd (Σ) and hence over K. On the other hand, since every element of Kαk (S) is algebraic over K(ζ1 , . . . , ζs ), every element of αd (K)αk+d (S) is algebraic over αd (K)(αd (ζ1 ), . . . , αd (ζs ) ) and hence over K. Since trdegK Kαk+q (S) = s, the elements αd (ζ1 ), . . . , αd (ζs ) are algebraically independent over K (actually, ∗

4.2. DIMENSION POLYNOMIALS OF σ-FIELD EXTENSIONS

255

they form a transcendence basis of Kαk+d (S) over K). We have arrived at a contradiction that implies the algebraic dependence of ζ1 , . . . , ζs over K ∗ and  hence the equality s ≥ trdegK ∗ L∗ . Corollary 4.2.20 Let K be an ordinary difference field with a basic set σ = {α}, K{y} the ring of σ-polynomials in one σ-indeterminate y over K, and L an overfield of K. Let an element η ∈ L be σ-algebraic over K and let r ∈ N be the smallest integer such that there is a σ-polynomial A ∈ K{y} of effective  order r that vanishes at η. Then trdegK ∗ (Kη)∗ = r. PROOF. Let s = trdegK ∗ (Kη)∗ . It follows from our considerations before Proposition 4.1.19 that trdegK Kαi (η) ≥ s for every i ∈ N, and the equality occurs for all sufficiently large i. Therefore, (see Corollary 4.1.18), s is the minimum of the orders of nonzero σ-polynomials C ∈ K{y} such that some αk (η) (k ∈ N) is a solution of C. Now, in order to complete the proof, i.e., to show that r = s, it remains to notice that K{y} contains a nonzero σ-polynomial C of effective order m with solution η if and only if K{y} contains a nonzero σ-polynomial C˜ of order m ˜ k (η)) = 0 for some k ∈ N. Indeed, given C with the described such that C(α properties, let ord C = m + q (q ≥ 0) and let C˜ be the σ-polynomial obtained from C by replacing each αi (y) (i = q, q + 1, . . . ) with αi−q (y). Clearly, ord C˜ = ˜ k (η)) = ˜ q (η)) = 0. Conversely, if 0 = C˜ ∈ K{y}, ord C˜ = m and C(α m and C(α 0 for some k ∈ N, then one can consider a σ-polynomial C obtained from C˜ by replacing each αi (y) (i = 0, 1, . . . ) with αi+k (y). Clearly, Eord C = m and C(η) = 0. 

4.2

Dimension Polynomials of Difference and Inversive Difference Field Extensions

In this section we introduce and study certain numerical polynomials associated with finitely generated difference and inversive difference field extensions. Properties of these polynomials give us an important technique for the study of difference fields and system of algebraic difference equations. Furthermore, as we shall see, such polynomials carry invariants of difference and inversive difference field extensions, that is, numbers that do not depend on the systems of generators of extensions. Unless otherwise is indicated, we assume that all fields considered below have zero characteristic. The following theorem is a difference version of the classical Kolchin’s result on differential dimension polynomial [103]. Theorem 4.2.1 Let K be a difference field with a basic set σ = {α1 , . . . , αn }, T the free commutative semigroup generated by σ, and for any r ∈ N, T (r) = {τ ∈ T |ord τ ≤ r}. Furthermore, let L = Kη1 , . . . , ηs  be a σ-overfield of K generated by a finite family η = {η1 , . . . , ηs }. Then there exists a polynomial φη|K (t) ∈ Q[t] with the following properties.

256

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

(i) φη|K (r) = trdegK K({τ ηj |τ ∈ T (r), 1 ≤ j ≤ s}) for all sufficiently large r ∈ N. (ii) deg φη|K (t) ≤ n and the polynomial φη|K (t) can be written as φη|K (t) =

n 

 t+i i

ai

i=0

where a0 , . . . , an ∈ Z. (iii) The integers an , d = deg φη|K (t) and ad are invariants of the polynomial φη|K (t), that is, they do not depend on the choice of a system of σ-generators η. Furthermore, an = σ-trdegK L. (iv) Let P be the defining σ-ideal of (η1 , . . . , ηs ) in the ring of σ-polynomials K{y1 , . . . , ys } and let A be a characteristic set of P with respect to some orderly ranking of {y1 , . . . , ys }. Furthermore, for every j = 1, . . . , s, let Ej = {(k1 , . . . , kn ) ∈ Nn |α1k1 . . . αnkn yj is the leader of a σ-polynomial in A}. Then φη|K (t) =

s 

ωEj (t)

i=1

where ωEj (t) is the Kolchin polynomial of the set Ej . PROOF. As in Section 2.4, let T Y denote the set of all terms τ yi (τ ∈ T, 1 ≤ i ≤ s), and let V = {u ∈ T Y | u is not a transform of any leader uA of a σ-polynomial A ∈ A}. Furthermore, for every r ∈ N, let V (r) = {u ∈ V | ord u ≤ r}. If A ∈ A, then A(η) = 0, hence uA (η) is algebraic over the field K({τ ηj | τ yj < uA (τ ∈ T, 1 ≤ j ≤ s)}). (The symbol < denotes our orderly ranking of {y1 , . . . , ys }.) It follows that for every r ∈ N, the field Lr = K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ s}) is an algebraic extension of the field K({v(η) | v ∈ V (r)}). By Proposition 2.4.4, the ideal P does not contain nonzero difference polynomials reduced with respect to A. It follows that for every r ∈ N, the set Vη (r) = {v(η) | v ∈ V (r)} is algebraically independent over K, hence Vη (r) is a transcendence basis of Lr over K and trdegK Lr = Card Vη (r). For every j = 1, . . . , s, kn the number of terms α1k1 . . . α nnyj in V (r) is equal to the number of n-tuples n (k1 , . . . , kn ) ∈ N such that i=1 ki ≤ r and (k1 , . . . , kn ) does not exceed any n-tuple in Ej with respect to the product order on Nn . By Theorem 1.5.2, for all sufficiently large r ∈ N this number is equal to ωEj (r) where ωEj (t) is the s  ωEj (r) Kolchin polynomial of the set Ej . Thus, trdegK Lr = Card Vη (r) = for all sufficiently large r ∈ N, so the numerical polynomial φη|K (t) =

j=1 s  i=1

ωEj (t)

satisfies conditions (i), (ii) and (iv) of the theorem. (Since degωEj ≤ n for j = 1, . . . , s, deg φη|K ≤ n, so that statement (ii) is a direct consequence of Corollary 1.4.5.)

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257

Let us show that if we represent the polynomial φη|K (t) in the form φη|K (t) = 

t+i with a0 , . . . , an ∈ Z, then an , d = deg φη|K and ad do not deai i i=0 pend on the choice of a system of σ-generators η of L over K. Indeed, let ζ = {ζ1 , . . . , ζq } be another system of σ-generators of L/K and let φζ|K (t) = 

n  t+i (b0 , . . . , bn ∈ Z) be the numerical polynomial such that φζ|K (r) = bi i i=0 trdegK K({τ ζj |τ ∈ T (r), 1 ≤ k ≤ q}) for all sufficiently large r ∈ N. Then there exists h ∈ N such that ηj ∈ K({τ ζk | τ ∈ T (h), 1 ≤ k ≤ q}) for j = 1, . . . , s and ζk ∈ K({τ ηj | τ ∈ T (h), 1 ≤ j ≤ s}) for k = 1, . . . , q. It follows that φη|K (r) ≤ φζ|K (r + h) and φζ|K (r) ≤ φη|K (r + h) for all sufficiently large r ∈ N whence the polynomials φη|K (t) and φζ|K (t) have the same degrees and the same leading coefficients. Now, in order to complete the proof we need to show that an is equal to the difference transcendence degree of L/K. Let e = σ-trdegK L. Without loss of generality we can assume that η1 , . . . , ηe form a σ-transcendence basis of L 1 ≤ j ≤e} is algebraically over K. Then for any r ∈ N, the set {τ ηj | τ ∈ T (r), r+n for all sufficiently independent over K hence φη|K (r) ≥ e Card T (r) = e n large r ∈ N. Therefore, an ≥ e. Let us prove the opposite inequality (and therefore, the desired equality an = e). For every j = e + 1, . . . , s, the element ηj is σ-algebraic over the σfield Kη1 , . . . , ηe , so there exists a term vj = τj yj (τj ∈ T ) such that τj ηj is algebraic over the field Kη1 , . . . , ηe ({τ ηi | τ ∈ T, e + 1 ≤ i ≤ s and τ yi < vj }). Therefore, there exists h ∈ N such that the elements
τe+1 ηe+1 , . . . , τs ηs are algebraic over the field K({τ ηi | τ ∈ T (h), 1 ≤ i ≤ e} {τ ηi | τ ∈ T, e+1 ≤ i ≤ s and τ yi < vj }). Let rj = ord τj (e + 1 ≤ j ≤ s), let τ  yj be a transform of vj and r = ord τ  . Then the element τ  ηj is algebraic over the field K({τ ηi | τ ∈
 T (h + r − rj ), 1 ≤ i ≤ e} {τ ηi | τ ∈ T, e + 1 ≤ i ≤ s and τ yi < τ  yj }). It follows that for all r ≥ max{re+1 , . . . , rs }, every element of the field Lr = over the field Lr = K({τ ηi | τ ∈ K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ s}) is algebraic
s T (r+h), 1 ≤ i ≤ e}∪{τ ηj | τ ∈ T (r)\ j=e+1 T (r−rj )τ
j }). Therefore, φη|K (r) = trdegK Lr ≤ trdegK K({τ ηi | τ ∈ T (r + h), 1 ≤ i ≤ e} {τ ηj | τ ∈ T (r), e + 1 ≤ s  j ≤ s}) = trdegK Lr ≤ e · Card T (r + h) + [Card T (r) − Card T (r − rj )] = n 

j=d+1

    

s r + n − rj r+n r+h+n − + for all sufficiently large r ∈ N. e n n n j=d+1

Since the last sum is a polynomial of r of degree less than n, we have an ≤ e.  Thus, an = σ-trdegK L. Definition 4.2.2 The polynomial φη|K (t) whose existence is established by Theorem 4.2.1 is called the difference (or σ-) dimension polynomial of the difference field extension L of K associated with the system of σ-generators η. The integers d = deg φη|K (t) and ad are called, respectively, the difference (or σ-)

258

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

type and typical difference (or σ-) transcendence degree of L over K. These invariants of φη|K (t) are denoted by σ-typeK L and σ-t.trdegK L, respectively. Example 4.2.3 With the notation of Theorem 4.2.1, suppose that the elements η1 , . . . , ηs are σ-algebraically independent over K. If φη|K (t) is the corresponding difference dimension polynomial of L/K, then φη|K (r)  = r+n for trdegK K({τ ηj | τ ∈ T, 1 ≤ j ≤ s}) = s · Card T (r) = s n all sufficiently  r ∈ N (see formula (1.4.16)). Therefore, in this case

large t+n . φη|K (t) = s n Theorem 4.2.4 Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let L be a finitely generated σ-field extension of K with a set of σ-generators η = {η1 , . . . , ηs }. σ-transcendence basis of L over (i) If d = σ-trdegK L and {η1 , . . . , ηd } is a  t+n . ( is the natural order K, then φ(ηd+1 ,...,ηs )|K η1 ,...,ηd  (t)  φη|K (t) − d n on the set of numerical polynomials introduced in Definition 1.5.10.) 

t+n for some m ∈ N if and only if (ii) φη|K (t) = m n σ-trdegK L = trdegK K(η1 , . . . , ηs ) = m. PROOF. For every r ∈ N, let A1 (r) = {τ ηj | τ ∈ T (r), 1 ≤ j ≤ d}, A2 (r) = {τ ηj | τ ∈ T (r), d + 1 ≤ j ≤ s}, and A(r) = A1 (r) ∪ A2 (r) = {τ ηj | τ ∈ T (r), 1 ≤ j ≤ s}. Then the definition of the polynomial φ(ηd+1 ,...,ηs ) | K η1 ,...,ηd  and Proposition 1.6.31 show that φ(ηd+1 ,...,ηs )|K η1 ,...,ηd  (r)

= trdegK η1 ,...,ηd  Kη1 , . . . , ηd (A2 (r)) ≤ trdegK(A1 (r)) K(A(r)) = trdegK K(A(r)) − trdegK K(A1 (r)) 

r+n = φη | K (r) − d n

for all sufficiently large r ∈ N. Therefore,

 t+n . n

φ(ηd+1 ,...,ηs )|K η1 ,...,ηd  (t)  φη|K (t) − d



t+n for some m ∈ N. Then m = σ-trdegK L n and without loss of generality we can assume that elements η1 , . . . , ηm form a σ-transcendence basis of L over K. In this case, φ(ηm+1 ,...,ηs ) | K η1 ,...,ηm  (t) = 0, since φ(ηm+1 ,...,ηs ) | K η1 ,...,ηm  (r) = trdegK K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ s}) −  

r+n r+n = 0 for all −m trdegK K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ m}) = m n n sufficiently large r ∈ N. Suppose that φη|K (t) = m

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259

Let P be the defining ideal of the (s − m)-tuple (ηm+1 , . . . , ηs ) in the ring of σ-polynomials Kη1 , . . . , ηm {y1 , . . . , ys−m } over the σ-field Kη1 , . . . , ηm  and let A be a characteristic set of P with respect to some orderly ranking < of y1 , . . . , ys−m such that y1 < y2 < · · · < ys−m . For every j = 1, . . . , s − m, let Ej denote the set of all n-tuples (k1 , . . . , kn ) ∈ Nn such that the term α1k1 . . . αnkn yj is a leader of some (obviously, unique) σ-polynomial in A. By Theorem 4.2.1(iv), s−m  ωEj (t) = 0, hence ωEj (t) = 0 for j = 1, . . . , φ(ηm+1 ,...,ηs ) | K η1 ,...,ηm  (t) = j=1

s − m. It follows that every set Ej (1 ≤ j ≤ s − m) consists of a single element (0, . . . , 0) (see Theorem 1.5.2). Since y1 < yj for j = 2, . . . , s−m, a σ-polynomial in A with the leader y1 is a usual polynomial in one indeterminate y1 with coefficients in Kη1 , . . . , ηm . It follows that ηm+1 , as well as any element τ ηm+1 (τ ∈ T ), is algebraic over Kη1 , . . . , ηm . Since φ(ηm+2 ,...,ηs ) | K η1 ,...,ηm  (t)  φ(ηm+1 ,...,ηs ) | K η1 ,...,ηm  (t), φ(ηm+2 ,...,ηs ) | K η1 ,...,ηm  (t) = 0, so one can repeat the above reasoning and obtain that all elements τ ηj with τ ∈ T, m + 1 ≤ j ≤ s are algebraic over Kη1 , . . . , ηm . Since ηm+1 , . . . , ηs are algebraic over Kη1 , . . . , ηm , there exists r0 ∈ N such that ηj (m + 1 ≤ j ≤ s) are algebraic over the field K({τ ηi | τ ∈ T (r0 ), 1 ≤ i ≤ m}). Therefore, for every r ≥ r0 , the field K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ s}) is algebraic over K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ m}). Suppose that ηm+1 is not algebraic over K(η1 , . . . , ηm ). Let p be the minimal number in the set of all q ∈ N such that ηm+1 is algebraic over the field K({τ ηj | τ ∈ T (q), 1 ≤ j ≤ m}) (according to our assumption, p ≥ 1). Since ηm+1 is transcendental over K({τ ηj | τ ∈ T (p − 1), 1 ≤ j ≤ m}), there exists an element v = τ0 ηh (1 ≤ h ≤ m) such that ord τ0 = p and v is algebraic over the field K({τ ηj | τ ∈ T (p), 1 ≤ j ≤ m} ∪ {ηm+1 } \ {v}) (see Theorem 1.6.26 and Proposition 1.1.6). It is easy to see that if τ  ∈ T and ord τ  = r ≥ r0 , then the element τ  v = τ  τ0 ηh is algebraic over the field K({τ ηj | τ ∈ T (r + p), 1 ≤ j ≤ m} ∪ {τ  ηm+1 } \ {τ  v}). Furthermore, τ  ηm+1 is algebraic over K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ m}) and therefore, over K({τ ηj | τ ∈ T (r + p), 1 ≤ j ≤ m}), whence τ  v is algebraic over K({τ ηj | τ ∈ T (r + p), 1 ≤ j ≤ m} \ {τ  v}). Thus, the set {τ ηj | τ ∈ T (r + p), 1 ≤ j ≤ m} is algebraically dependent over K that contradicts the fact that η1 , . . . , ηm are σ-algebraically independent over K. It follows that ηm+1 is algebraic over the field K(η1 , . . . , ηm ) and similarly every element ηm+2 , . . . , ηs is algebraic over K(η1 , . . . , ηm ), so that m = σtrdegK L = trdegK K(η1 , . . . , ηs ). Conversely, suppose that the last equality holds and {η1 , . . . , ηm } is a σtranscendence basis of Kη1 , . . . , ηs  over K. Then the elements η1 , . . . , ηm are algebraically independent over K and K(η1 , . . . , ηs ) is an algebraic extension of K(η1 , . . . , ηm ). It follows that if τ ∈ T (r) (r ∈ N) and 1 ≤ i ≤ s, then the element τ ηi is algebraic over the field K({τ ηj | τ ∈ T (r), 1 ≤ j ≤ m}). Therefore, trdegK K({τ t+n1 ≤ j ≤  ηj | τ ∈ T (r), 1 ≤ j ≤ s}) = trdegK K({τ ηj | τ ∈ T (r), for all sufficiently large r ∈ N, so that φ  (t) = m m}) = m t+n η|K n n . The following theorem gives versions of Theorem 4.2.1 for finitely generated inversive difference field extensions.

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

260

Theorem 4.2.5 Let K be an inversive difference field with a basic set σ = generated by σ and for any r ∈ {α1 , . . . , αn }, Γ the free commutative group  n N, Γ(r) = {γ = α1k1 . . . αnkn ∈ Γ|ord γ = i=1 |ki | ≤ r}. Furthermore, let ∗ ∗ L = Kη1 , . . . , ηs  be a σ -overfield of K generated by a finite family η = {η1 , . . . , ηs }. Then there exists a polynomial ψη|K (t) ∈ Q[t] with the following properties. (i) ψη|K (r) = trdegK K({γηj |γ ∈ Γ(r), 1 ≤ j ≤ s}) for all sufficiently large r ∈ N. (ii) deg ψη|K (t) ≤ n and the polynomial ψη|K (t) can be written as ψη|K (t) =

2n a n t + o(tn ) n!

(4.2.3)

where a ∈ Z and o(tn ) is a numerical polynomial of degree less than n. (iii) The integers a, d = deg ψη|K (t) and the coefficient of td in the polynomial ψη|K (t) do not depend on the choice of a system of σ-generators η. Furthermore, a = σ-trdegK L. (iv) Let P be the defining σ-ideal of (η1 , . . . , ηs ) in the ring of σ ∗ -polynomials K{y1 , . . . , ys }∗ and let A be a characteristic set of P with respect to some orderly ranking of {y1 , . . . , ys }. Furthermore, for every j = 1, . . . , s, let Fj = {(k1 , . . . , kn ) ∈ Zn |α1k1 . . . αnkn yj is a leader of a σ ∗ -polynomial in A}. Then s  ψη|K (t) = φFj (t) where φFj (t) is standard Z-dimension polynomial of the i=1

set Fj (see Definition 1.5.18). PROOF. Let K{y1 , . . . , ys }∗ be the ring of inversive difference (σ ∗ -) polynomials over K and let ≤ be an orderly ranking of the family of σ ∗ -indeterminates y1 , . . . , ys (see Definition 2.4.6 and the terminology introduced after the definition). Furthermore, let W denote the set of all terms γyj (γ ∈ Γ, 1 ≤ j ≤ s) which are not transforms of any leader uA of a σ ∗ -polynomial A ∈ A. (The concept of transform is understood in the sense of Definition 2.4.5.) As in the proof of Theorem 4.2.1 we obtain that for any r ∈ N, the set W (r) = {γyj ∈ W | ord γ ≤ r, 1 ≤ j ≤ s} is a transcendence basis of the field K({γηj γ ∈ Γ(r), 1 ≤ j ≤ s}) over K. Furthermore, for every j = 1, . . . , s and for every r ∈ N, the number of terms α1k1 . . . αnkn yj inW (r) is equal to n the number of n-tuples (k1 , . . . , kn ) ∈ Zn \ Fj such that i=1 |ki | ≤ r and (k1 , . . . , kn ), is not greater than any element of Fj with respect to the order
introduced in Section 1.5. It follows from Theorem 1.5.17 that the polynomial s  φFj (t) satisfies condition (i) of the theorem, deg ψη|K ≤ n, and ψη|K (t) = j=1

statement (iv) holds. Furthermore, the polynomial ψη|K can be written in the form (4.2.3) with some a ∈ Q. The first part of (iii) can be easily obtained by repeating the proof of the corresponding part of Theorem 4.2.1(iii). Let us show that if ψη|K (t) is written in the form (4.2.3), then a is the difference (σ-) transcendence degree of L over

4.2. DIMENSION POLYNOMIALS OF σ-FIELD EXTENSIONS

261

K (in particular, a is an integer). Let d = σ-trdegK L. Without loss of generality we can assume that η1 , . . . , ηd form a σ-transcendence basis of L/K, so that ψη|K (r)

≥

trdegK K({γyj | γ ∈ Γ(r), 1 ≤ j ≤ d}) = d Card Γ(r) 

 n  n r+i (−1)n−i 2i = d i i i=1

for all sufficiently large r ∈ N (see formula (1.4.17)). It follows that a = ∆n ψη|K (t) ≥ d (as before, ∆n f (t) denotes the nth finite difference of a poly2n nomial f (t), see Section 1.4). To prove the opposite inequality let us denote the kth ortant of Zn by (we used this notation in Section 1.5) and set Γk = {γ = α1l1 . . . αnln ∈ 2n  (n) Γ | (l1 , . . . , ln ) ∈ Zk } (clearly, Γ = Γk ). Since the elements ηd+1 , . . . , ηs

(n) Zk

i=1

are σ-algebraic over Kη1 , . . . , ηd , for every j = d + 1, . . . , s and for every (k) (k) k = 1, . . . , 2n , there exists an element γj ∈ Γk such that γj ηj is algebraic (k)

over Kη1 , . . . , ηd ({γηj | γ ∈ Γk , γyj < γj yj }). Therefore, there is an element (k)

r0 ∈ N such that γj ηj is algebraic over the field K({γηi | γ ∈ Γ(r0 ), 1 ≤ i ≤
(k) d} {γηj | γ ∈ Γk , γyj < γj yj }) for j = d + 1, . . . , s. (k)

Let rjk = ord γj

(d + 1 ≤ j ≤ s, 1 ≤ k ≤ 2n ) and let γ  yj (γ ∈ Γk , (k)

d + 1 ≤ j ≤ s) be a transform of γj yj (in the sense of Definition 2.4.5). Clearly, if r = ord γ  , then r ≥ rjk and the element γ  ηj is algebraic over the field K({γηi | γ ∈ Γ(r0 + r − rjk ), 1 ≤ i ≤ d} ∪ {γηj | γ ∈ Γ, γyj < γ  yj }). Thus, for any r ≥ maxj,k {rjk }, every element of the field K({γηi | γ ∈ Γ(r), 1 ≤ j ≤ s}) is algebraic over the field K({γηi | γ ∈ Γ(r0 + r), 1 ≤ i ≤ d}



2  n

{γηj | γ ∈ Γ(r) \

(k)

Γk (r − rjk )γj ,

k=1

d + 1 ≤ j ≤ s}) where Γk (q) (q ∈ N) denotes the set {γ ∈ Γk | ord γ ≤ q}.

Now, repeating the arguments of the proof of part (iii) of Theorem 4.2.1, we obtain that ψη|K (r)

= trdegK K({γηj |γ ∈ Γ(r), 1 ≤ j ≤ s}) ≤ d · Card Γ(r + r0 ) 2 s   n

+

j=d+1 k=1

! " (k) Card Γk (r) − Γk (r − rj )

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

262

(k)

(k)

r − rj + n n

for all sufficiently large r ∈ N. Since Card(Γk (r − rj ) =

 (see

formula (1.4.16)), ψη|K (t) ≤



 n t+i i i i=1  n  

 s 2 (k)   t − rj + n t+n + − + o(tn ). n n d Card Γ(r)  d

n 

(−1)n−i 2i

j=d+1 k=1

∆n ψ

(t)

η|K ≥ d, imply The last inequality and the already proven inequality a = 2n ∗ that a = d = σ -trdegK L and the polynomial ψη|K (t) can be written in the form (4.2.3). This completes the proof of the theorem. 

Definition 4.2.6 The polynomial ψη|K (t) whose existence is established by Theorem 4.2.5 is called the σ ∗ -dimension polynomial of the σ ∗ -field extension L of K associated with the system of σ ∗ -generators η. Example 4.2.7 With the notation of Theorem 4.2.5, suppose that the elements η1 , . . . , ηs are σ-algebraically independent over K, and let φη|K (t) denote the corresponding σ ∗ -dimension polynomial of L/K. As in Example 4.2.3, we obtain that 

 n  n r+k (−1)n−k 2k ψη|K (r) = s · Card Γ(r) = s k k k=0

for all sufficiently large r ∈ N (see formula (1.4.17)), whence ψη|K (t) = s

n 

(−1)

k=0



 n t+k . 2 k k

n−k k

Now, with the notation of Theorem 4.2.5, we are going to show that the module of differentials ΩL|K is a vector σ ∗ -L-space and the σ ∗ -dimension polynomial ψη|K (t) of the extension L/K coincides with the σ ∗ -dimension polynomial associated with the natural excellent filtration of ΩL|K . First, we need the following lemma. Lemma 4.2.8 Let A be an inversive difference ring with a basis set σ and let B be a σ ∗ -A-algebra (see Definition 2.2.8). Then the module of differential ΩB|A has a canonical structure of a σ ∗ -B-module such that for every α ∈ σ ∗ and b ∈ B, α(db) = dα(b). This structure is unique. PROOF. The uniqueness is clear since the B-module ΩB|A is generated by the elements db for b ∈ B. To show the existence of the of a  desired structure ∗ on the B-module Ω , observe that B B is a σ -A-algebra σ ∗ -B-module B|A A    (where α(x y)  = α(x) α(y) for  every α ∈ σ ∗ and every generator x y of the A-algebra B A B) and µ : B A B → B (x y → xy for x, y ∈ B) is

4.2. DIMENSION POLYNOMIALS OF σ-FIELD EXTENSIONS

263

 Ker µ is a σ ∗ -ideal of B A B, a σ-epimorphism of σ ∗ -A-algebras. Then I =  2 ∗ and if b ∈ B, then α((b 1 − 1 b) + I 2 ) = (α(b) 1 − I/I  is a σ -B-module 2 1 α(b)) + I for every α ∈ σ ∗ . The last formula is equivalent to the equality α(db) = dα(b) and this completes the proof.  Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let L be a finitely generated σ ∗ -field extension of K with a set of generators η = {η1 , . . . , ηs }. Let ΩL|K be a vector σ ∗ -space of differentials (with the structure defined in the last lemma) and for every r ∈ N, let (ΩL|K )r denote the vector L-subspace of ΩL|K generated by the set {dγ(ηi )|γ ∈ Γ(r), 1 ≤ i ≤ s}. Furthermore, let (ΩL|K )r = 0 for any r ∈ Z, r < 0. Theorem 4.2.9 With the above notation, (i) ((ΩL|K )r )r∈Z is an excellent filtration of the σ ∗ -L-module ΩL|K . (ii) dimK (ΩL|K )r = trdegK K({γηj |γ ∈ Γ(r), 1 ≤ j ≤ s}) for all r ∈ Z. (iii) The σ ∗ -dimension polynomial ψη|K (t) is equal to the σ ∗ -dimension polynomial of ΩL|K associated with the filtration ((ΩL|K )r )r∈Z . PROOF. Let E be the ring of σ ∗ -operators over L and (Er )r∈Z the standard filtration of E. Furthermore, let Lr = K({γηj |γ ∈ Γ(r), 1 ≤ j ≤ s}) (r ∈ N) and Lr = K for r ∈ Z, r < 0. Since α(dξ) = dα(ξ) for any ξ ∈ L, α ∈ σ ∗ and Lr+q = Lr ({γηj | γ ∈ Γ(q), 1 ≤ j ≤ s}) for all r, q ∈ N, ((ΩL|K )r )r∈Z is a filtration of the σ ∗ -L-module ΩL|K (see Definition 3.5.1) and Eq (ΩL|K )r = (ΩL|K )r+q for all r, q ∈ N. Since the finite set {γdηj | γ ∈ Γ(r), 1 ≤ j ≤ s} generates (ΩL|K )r as a vector L-space, the filtration ((ΩL|K )r )r∈Z is excellent. By Proposition 1.7.13, if {ξ1 , . . . , ξkr } (r ∈ N, kr ≥ 0) is a transcendence basis of the field Lr over K, then {dξ1 , . . . , dξkr } is a basis of the vector L-space  (ΩL|K )r . This implies statements (ii) and (iii). Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, L a finitely generated σ ∗ -field extension of K with a set of σ ∗ -generators η = {η1 , . . . , ηs }, and E the ring of σ ∗ -operators over L. Since the ring E is left Noetherian (see Theorem 3.4.2) and the left E-module ΩL|K is finitely generated, there exists a finite resolution dq−1

d

ρ

0 M0 − → ΩL|K 0 → Mq −−−→ Mq−1 → . . . −→

(4.2.4)

where each Mi (0 ≤ i ≤ q) is a free filtered E-module (with respect to the standard filtration of E) and ρ, d0 , . . . , dq−1 are σ-homomorphisms of filtered E-modules (see [176, Theorem 10.4.9]). As we have seen in Example 3.5.4, the σ ∗ -dimension polynomial χMi (t) of the module Mi (0 ≤ i ≤ q) can be expressed as one of the sums in formula (3.5.5). The last statement of Theorem 4.2.9 and the exact sequence (4.2.4) show that the σ ∗ -dimension polynomial ψη|K (t) of the extension L/K can be expressed as φη|K (t) =

q  i=1

(−1)i χMi (t).

(4.2.5)

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

264

  in the canonical Applying the last formula in (3.5.5) and writing each t+i−l i form (1.4.9) we can represent the polynomial ψη|K (t) in the form 

n  i t+i (4.2.6) ai 2 ψη|K (t) = i i=0

where a0 , . . . , an ∈ Z. The following definition is based on Theorem 4.2.5(iii) and the last observation. Definition 4.2.10 Let K be an inversive difference field with a basic set σ, L a σ ∗ -field extension of K generated by a finite family η = {η1 , . . . , ηs }, and ψη|K (t) the corresponding σ ∗ -dimension polynomial of the extension L/K written in the ∆d ψη|K (t) form (4.2.6). Then d = deg ψη|K and the coefficient ad = are called 2d ∗ the inversive difference (or σ -) type and typical inversive difference(or typical σ ∗ -) transcendence degree of L over K. They are denoted by σ ∗ -typeK L and σ ∗ t.trdegK L, respectively. (By Theorem 4.2.5, these characteristics do not depend on the choice of the set of σ ∗ -generators of L over K; if d = n, then ad = σtrdegK L.) The proof of Theorem 4.2.9 shows that one can naturally generalize Theorem 4.2.5 to inversive difference fields of arbitrary characteristic. Definition 4.2.11 Let K be an inversive difference field with a basic set of automorphisms σ = {α1 , . . . , αn } and let L be a σ ∗ -field extension of K. An ascending sequence {Lr }r∈Z of intermediate fields of L/K is called a filtration of this σ ∗ -field extension if it satisfies the following conditions: (i) If η ∈ Lr and α ∈ σ ∗ , then α(η) ∈ Lr+1 .  Lr = L. (ii) r∈Z

(iii) Lr = K for all sufficiently small r ∈ Z. A filtration {Lr }r∈Z of L/K is called excellent if every Lr is a finitely generated field extension of K (a filtration with this property is said to be finite) and there exists r0 ∈ Z such that Lr = Lr0 ({γ(η) | γ ∈ Γ(r − r0 ), η ∈ Lr0 }) for every r ∈ Z, r > r0 (a filtration with the last property is called good). Finally, a filtration {Lr }r∈Z of L/K is called separable if every Lr is a separable field extension of K. Theorem 4.2.12 Let K be an inversive difference field of arbitrary characteristic with a basic set of automorphisms σ = {α1 , . . . , αn }. Let L be a σ ∗ -field extension of K and let {Lr }r∈Z be an excellent and separable filtration of L/K. Then there exists a numerical polynomial ψ(t) in one variable t such that (i) ψ(r) = trdegK Lr for all sufficiently large r ∈ Z. (ii) deg ψ ≤ n and the polynomial ψ(t) can be represented as 

n  i t+i ψ(t) = ai 2 i i=0 where a0 , . . . , an ∈ Z.

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(iii) an = σ ∗ -trdegK L.Furthermore, if the degree d of the polynomial ψ(t) is less than n, then d and the coefficient ad do not depend on the excellent and separable filtration of L/K the polynomial ψ(t) is associated with. The proof of this theorem is similar to the proof of Theorem 4.2.9 (with the use of Proposition 1.7.13(iv) and Proposition 1.6.36). We leave the details to the reader as an exercise.  The polynomial whose existence is established by Theorem 4.2.12 is called a σ ∗ -dimension polynomial of the σ ∗ -field extension L/K associated with the given excellent and separable filtration of this extension. The following statement is an analog of Theorem 4.2.4 for inversive difference field extensions. The proof of this result is similar to the proof of Theorem 4.2.4, we leave the details to the reader an exercise. Theorem 4.2.13 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let L be a finitely generated σ ∗ -field extension of K with a set of σ ∗ -generators η = {η1 , . . . , ηs }. Then (i) If d = σ-trdegK L and {η1 , . . . , ηd } is a σ-transcendence basis of L over   . K, then ψ(ηd+1 ,...,ηs )|K η1 ,...,ηd  (t)  φη|K (t) − d t+n n 

 n  t+i n−i i n for some m ∈ N if and only if (ii) ψη|K (t) = m (−1) 2 i i i=0 σ-trdegK L = trdegK K(η1 , . . . , ηs ) = m.  Based on Theorem 3.5.15 and the corresponding result for differential field extensions of zero differential transcendence degree (see [97, Section 3, Corollary 2] or [110, Theorem 5.6.3]) one can expect that if L is a finitely generated σ ∗ -field extension of an inversive difference field K with a basic set σ = {α1 , . . . , αn }, then there exists a set σ1 = {β1 , . . . , βn } of pairwise commuting automorphisms of L such that σ1 is equivalent to σ (in the sense of Definition 3.5.14) and L is a finitely generated σ2∗ -field extension of K if L and K are treated as inversive difference fields with the basic set σ2 = {β1 , . . . , βn−1 }∗ . The following example shows that this is not so. Example 4.2.14 Let us consider the field of real numbers R as an inversive difference field whose basic set σ consists of the identical automorphism α. Let A be the ring of all functions f (x) (x ∈ R) with real values which are defined for all x ∈ R except for possibly finitely many points. Then A can be treated as a σ ∗ -overring of R such that αf (x) = f (x + 1) for every f (x) ∈ A. Let η denote x the function 22 ∈ A and let L = Rη∗ be the σ ∗ -field generated by η over R x+1 √ √ = η 2 , L = R(η, η, 4 η, . . . ) (clearly, L is a σ ∗ -subring of A). Since α(η) = 22 x−1 x−2 √ √ = α−1 (η), 4 η = 22 = α−2 (η), . . . . where η = 22 With the notation of Theorem 4.2.5 we have √ √ ψη | R (r) = trdegR R(η, η, . . . , 2r η) = 1

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for every r ∈ N, so ψη | R (t) = 1 and σ ∗ -trdegR L = 0. At the same time L is not a finitely generated field extension of R. (Otherwise, we would have √ √ L = R(η, η, . . . , 2s η) for some s ∈ N that contradicts the obvious fact that √ √ √ s+1 2 η∈ / R(η, η, . . . , 2s η).) We see that despite the equality σ ∗ -trdegR L = 0, L cannot be considered as a finitely generated inversive difference field extension of R with respect to an empty basic set. (Notice that the only sets of automorphisms that are equivalent to a basic set σ = {α} of an ordinary inversive difference field are σ and {α−1 }.) The last example shows that there is no direct analog of Theorem 3.5.16 for inversive difference field extensions. However, one can obtain the following weak version of this theorem. Theorem 4.2.15 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, let L be a finitely generated σ ∗ -field extension of K, and let d = σ ∗ -typeK L. Then there exists a set σ1 = {β1 , . . . , βn } of mutually commuting automorphisms of L and a finite family ζ = {ζ1 , . . . , ζq } of elements of L such that σ1 is equivalent to σ and if σ2 = {β1 , . . . , βd }, then L is an algebraic extension of the field H = Kζ1 , . . . , ζq ∗σ2 . (The last field is a finitely generated σ2∗ -field extension of K when K is treated as an inversive difference field with the basic set σ2 .) PROOF. By Theorem 4.2.9, the module of differentials ΩL|K is a finitely generated vector σ ∗ -L-space and it(ΩL|K ) = d (in accordance with Definition 3.5.9, it(M ) denote the σ ∗ -type of a vector σ ∗ -L-space M ). By Theorem 3.5.16, there exists a set σ1 = {β1 , . . . , βn } of mutually commuting automorphisms of L and a finite family ζ = {ζ1 , . . . , ζq } of elements of L such that σ1 is equivalent to σ and the elements dζ1 , . . . , dζq generate ΩL|K as a vector σ2∗ -L-space, where σ2 = {β1 , . . . , βd }. If H = Kζ1 , . . . , ζq ∗σ2 and η ∈ L, then the element dη ∈ ΩL|K is a linear combination of elements dζ1 , . . . , dζq with coefficients in the ring of σ2∗ -operators over L, that is, dη is a linear combination of finitely many elements d(γij ζi ) (1 ≤ i ≤ q, 1 ≤ j ≤ ki for some k1 , . . . , kq ∈ N) where all γij belong to the free commutative group Γσ2 generated by σ2 . By Proposition 1.7.13, the element η is algebraic over the field K({γij ζi ) | 1 ≤ i ≤ q, 1 ≤ j ≤ ki }), hence it is algebraic over H. This completes the proof.  Theorem 4.2.1(iv) and Theorem 4.2.5(iv), together with Theorem 1.5.11(ii) and Proposition 1.5.19, show that the set of all dimension polynomials of finitely generated difference field extensions coincides with the set of all dimension polynomials of finitely generated difference field extensions; moreover, each of these sets coincides with the set W of all Kolchin polynomials. By Theorem 1.5.11(vi), the set W is well-ordered with respect to the order ≺ on the set of all numerical polynomials (see Definition 1.5.10). Let K be a difference field with a basic set σ and L a finitely generated σfield extension of K. Let Wσ (L, K) denote the set of all σ-dimension polynomials of the extension L/K associated with various finite systems of σ-generators of

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L over K. Since Wσ (L, K) ⊆ W , there exists a system of σ-generators η = {η1 , . . . , ηs } of L over K such that φη|K (t) is the minimal element of the set Wσ (L, K) (well-ordered by ≺). This σ-dimension polynomial φη|K (t) is called the minimal σ-dimension polynomial of the extension L/K; it is denoted by φL|K (t). Similarly, if K is an inversive difference field with a basic set σ and L a finitely generated σ ∗ -field extension of K, then the set of all σ ∗ -dimension polynomials of L over K, denoted by Wσ∗ (L, K), is a well-ordered subset of W (with respect to ≺). The minimal element of this set (which is a σ ∗ -dimension polynomial ψη|K (t) asociated with some finite system of σ ∗ -generators η of L over K) is called the minimal σ ∗ -dimension polynomial of the extension L/K; it is denoted by ψL|K (t). We conclude this section with two theorems on multivariable dimension polynomials of difference and inversive difference field extensions. Let K be a difference field of with a basic set σ = {α1 , . . . , αn } and let T be the free commutative semigroup generated by σ. As in Section 3.3, let a partition of the set σ into a disjoint union of its subsets be fixed: σ = σ 1 ∪ · · · ∪ σp

(4.2.7)

where p ∈ N, and σ1 = {α1 , . . . , αn1 }, σ2 = {αn1 +1 , . . . , αn1 +n2 }, . . . , σp = {αn1 +···+np−1 +1 , . . . , αn } (ni ≥ 1 for i = 1, . . . , p ; n1 + · · · + np = n). As in section 3.3, for any element τ = α1k1 . . . αnkn ∈ T we consider the orders of τ n1 +···+ni kν with n0 = 0) and with respect to each set σi (ordi τ = ν=n 1 +···+ni−1 +1 set T (r1 , . . . , rp ) = {τ ∈ T |ord1 τ ≤ r1 , . . . , ordp τ ≤ rp } for any r1 , . . . , rp ∈ N. Also, if Σ is any subset of Np , we set Σ = {e ∈ Σ|e is a maximal element of Σ with respect to one of the p! lexicographic orders <j1 ,...,jp where j1 , . . . , jp is a permutation of 1, . . . , p}. Theorem 4.2.16 With the above notation, let L = Kη1 , . . . , ηs  be a difference (σ-) field extension of K generated by a finite set η = {η1 , . . . , ηs }. Then there exists a polynomial φη|K (t1 , . . . , tp ) in p variables with rational coefficients such that (i) φη (r1 , . . . , rp ) = trdegK K({τ ηi | τ ∈ T (r1 , . . . , rp ), 1 ≤ j ≤ s}) for all sufficiently large (r1 , . . . , rp ) ∈ Np (ii) degti φη ≤ ni (1 ≤ i ≤ p), so that deg φ ≤ n and the polynomial φη (t1 , . . . , tp ) can be represented as φη (t1 , . . . , tp ) =

n1  i1 =0

...

np  ip =0

ai1 ...ip

  t1 + i1 tp + ip ... i1 ip

where ai1 ...ip ∈ Z for all i1 , . . . , ip . (iii) d = deg φη , an1 ...np , p-tuples (j1 , . . . , jp ) ∈ Σ , the corresponding coefficients aj1 ...jp , and the coefficients of the terms of total degree d do not depend on the choice of the system of σ-generators η of L over K. Furthermore, an1 ...np = σ-trdegK L.

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Theorem 4.2.17 Let K be an inversive difference field and let a partition (4.2.7) of its basic set σ = {α1 , . . . , αn } be fixed. Let Γ be the free commutative group generated by σ and for any r1 , . . . , rn ∈ N let Γ(r1 , . . . , rp ) = {γ ∈ Γ|ord1 τ ≤ r1 , . . . , ordp τ ≤ rp } ( ordi γ is the order of an element γ with respect to the set σi defined in the last part of Section 3.5). Furthermore, let L = Kη1 , . . . , ηs  be a σ ∗ -field extension of K generated by a finite set η = {η1 , . . . , ηs }. Then there exists a polynomial ψη|K (t1 , . . . , tp ) in p variables with rational coefficients such that (i) ψη (r1 , . . . , rp ) = trdegK K({γηi | γ ∈ Γ(r1 , . . . , rp ), 1 ≤ j ≤ s}) for all sufficiently large (r1 , . . . , rp ) ∈ Np . (ii) degti ψη ≤ ni (1 ≤ i ≤ p), so that deg ψ ≤ n and the polynomial ψη (t1 , . . . , tp ) can be represented as

  np n1   t1 + i1 tp + ip ... ... ai1 ...ip ψη (t1 , . . . , tp ) = i1 ip i =0 i =0 1

p

where ai1 ...ip ∈ Z for all i1 , . . . , ip . (iii) d = deg ψη , an1 ...np , p-tuples (j1 , . . . , jp ) ∈ Σ , the corresponding coefficients aj1 ...jp , and the coefficients of the terms of total degree d do not depend on the choice of the system of σ ∗ -generators η of L over K. Furthermore, an1 ...np = σ-trdegK L. 2n | an1 ...np and 2n Definition 4.2.18 Numerical polynomials φη (t1 , . . . , tp ) and ψη (t1 , . . . , tp ) whose existence is established by Theorems 4.2.16, 4.2.17, are called dimension polynomials of the σ- (respectively, σ ∗ -) field extension L/K associated with the given system of σ- (respectively, σ ∗ -) generators η and with the given partition of the basic set σ into p disjoint subsets σ1 , . . . , σp . PROOF OF THEOREMS 4.2.16 and 4.2.17. Let ΩL|K be the module of differentials associated with the extension L/K described in Theorem 4.2.17 and let for any r1 , . . . , rp ∈ N, (ΩL|K )r1 ,...,rp denote the vector L-subspace of ΩL|K generated by the set {dγ(ηi ) | γ ∈ Γ(r), 1 ≤ i ≤ s}. Furthermore, let (ΩL|K )r1 ,...,rp = 0 whenever ri < 0 for at least one i. By Lemma 4.2.8, ΩL|K is a vector σ ∗ -L-space, and it is easy to see (cf. the proof of Theorem 4.2.9) that {(ΩL|K )r1 ,...,rp | (r1 , . . . , rp ) ∈ Zp } is an excellent p-dimension filtration of ΩL|K and dimL (ΩL|K )r1 ,...,rp = trdegK K({γηi | τ ∈ Γ(r1 , . . . , rp ), 1 ≤ j ≤ s}) for all r1 , . . . , rp ∈ Z. Now all statements of Theorem 4.2.17 follow from Theorem 3.5.38. In order to prove Theorem 4.2.16, we consider the following generalization of the method of characteristic sets used in the proof of Theorem 4.2.1. Let K{y1 , . . . , ys } be the ring of difference (σ-) polynomials in σ-indeterminates y1 , . . . , ys over K and let T Y denote the set of all elements τ yi (τ ∈ T, 1 ≤ i ≤ s) called terms. Let us consider p orders <1 , . . . ,

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269

if τ
(1) d−1

+ · · · + I0 (σ-polynomials Id , Id−1 , . . . , I0 do not A = Id (uA ) + Id−1 (uA ) (1) contain uA ), then Id is called the leading coefficient of A; it is denoted by IA . Let A and B be two σ-polynomials in K{y1 , . . . , yn }. We say that A has lower rank then B and write rk A < rk B if either A ∈ K, B ∈ / K, or the vec(1) (2) (p) (1) tor (uA , degu(1) A, ord2 uA , . . . , ordp uA ) is less than the vector (uB , degu(1) B, (2)

A

B

(p)

(1)

ord2 uB , . . . , ordp uB ) with respect to the lexicographic order (where uA and (1) uB are compared with respect to <1 and all other coordinates of the vectors are compared with respect to the natural order on N). If the two vectors are equal (or A ∈ K and B ∈ K) we say that the σ-polynomials A and B are of the same rank and write rk A = rk B. A σ-polynomial B is said to be reduced with respect to a σ-polynomial A if the following two conditions hold. (1)

(i)

(i) B does not contain any term τ uA (τ ∈ T, τ = 1) such that ordi (τ uA ) ≤ (i) ordi uB for i = 2, . . . , p. (1)

(ii) If B contains uA , then either there exists j, 2 ≤ j ≤ p, such that (j) (j) (j) (j) ordj uB < ordj uA or ordj uA ≤ ordj uB for all j = 2, . . . , p and degu(1) B < A degu(1) A. A

A σ-polynomial B is said to be reduced with respect to a set Σ ∈ K{y1 , . . . , ys } if B is reduced with respect to every element of Σ.  A set of ∆-polynomials Σ ⊆ K{y1 , . . . , yn } is called autoreduced if Σ K = ∅ and every element of Σ is reduced with respect to any other element of this set. It follows from Lemma 1.5.1 that if S is any infinite set of terms τ yj (τ ∈ T, 1 ≤ j ≤ s), then there exists an index j (1 ≤ j ≤ s) and an infinite sequence of terms τ1 yj , τ2 yj , . . . , τk yj , . . . such that τk |τk+1 for all k = 1, 2, . . . . This fact, in turn, implies that every autoreduced set is finite. Indeed, suppose that Σ is an infinite autoreduced subset of K{y1 , . . . , ys }. Then Σ must contain an infinite set Σ ⊆ Σ such that all σ-polynomials in Σ have different 1-leaders. (If it is not so, then there exists an infinite set Σ1 ⊆ Σ such that all σ-polynomials from Σ1 have the same 1-leader u. By Lemma 1.5.1, (2) (p) the infinite set {(ord2 uA , . . . , ordp uA |A ∈ Σ1 } contains a nondecreasing (2)

(p)

(2)

(p)

infinite sequence (ord2 uA1 , . . . , ordp uA1 ) ≤P (ord2 uA2 , . . . , ordp uA2 ) ≤P . . .

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270

(A1 , A2 , · · · ∈ Σ1 , ≤P is the product order on Np ). Since the sequence of degrees {degu Ai |i = 1, 2, . . . } cannot be strictly decreasing, there exists two indices i and j such that i < j and degu Ai ≤ degu Aj . We obtain that Aj is reduced with respect to Ai that contradicts the fact that Σ is an autoreduced set.) Thus, one can assume that all 1-leaders of our infinite autoreduced set Σ are different. Then, as we have noticed, there exists an infinite sequence B1 , B2 , . . . (1) (1) (1) of elements of Σ such that uBi |uBi+1 for all i = 1, 2, . . . . Let kij = ordj uBi (i)

and lij = ordj uBi (2 ≤ j ≤ p). Obviously, lij ≥ kij (i = 1, 2, . . . ; j = 2, . . . , p), so that {(li2 − ki2 , . . . , lip − kip )|i = 1, 2, . . . } ⊆ Np−1 . By Lemma 1.5.1, there exists an infinite sequence of indices i1 < i2 < . . . such that (li1 2 −ki1 2 , . . . , li1 p − ≤P (li2 2 − ki⎞ , . . . , li2 p − ki2 p ) ≤P . . . . Then for any j = 2, . . . , p, we have ki1 p ) ⎛ 22 (1)

ordj ⎝

u Bi

2

(1)

u Bi

1

(j) (j) uBi ⎠ = ki2 j − ki1 j + li1 j ≤ ki2 j − li2 j − ki2 j = li2 j = ordj uBi , 1

2

(1)

(1)

(j)

so that Bi2 contains a term τ uBi = uBi such that τ = 1 and ordj (τ uBi ) ≤ (j)

1

2

1

ordj uBi for j = 2, . . . , p. Since the sequence of degrees of Bik with respect to (1)

2

uik cannot be infinite, Σ contains two σ-polynomials which are reduced with respect to each other, contrary to the fact that the set Σ is autoreduced. Thus, every autoreduced set is finite. Let {A1 , . . . , Ar } be an autoreduced set in the ring K{y1 , . . . , ys }, let Ik denote the initial of Ak with respect to the ranking <1 of T Y (1 ≤ k ≤ r), and let I(Σ) = {B ∈ K{y1 , . . . , ys } | either B = 1 or B is a product of finitely many elements of the form τ (Ik ), τ ∈ T, 1 ≤ k ≤ r}. Repeating the proof of Theorem 2.4.1, one can obtain that for any σ-polynomial B, there exist B0 ∈ K{y1 , . . . , ys } and J ∈ I(Σ) such that B0 is reduced with respect to Σ, B0 has lower rank than B (in the sense of Section 2.4 with the order <1 on the set T Y ), and JB ≡ B0 (mod[Σ]) (that is, JB − B0 ∈ [Σ]). As in Section 2.4, while considering autoreduced sets in the ring K{y1 ,. . . ,yn } we shall always assume that their elements are arranged in order of increasing rank. Given two autoreduced sets Σ = {A1 , . . . , Ar } and Σ = {B1 , . . . , Bs }, we say that Σ has lower rank than Σ if one of the following two cases holds. (1) There exists k ∈ N such that k ≤ min{r, s}, rk Ai = rk Bi for i = 1, . . . , k − 1 and rk Ak < rk Bk . (2) r > s and rk Ai = rk Bi for i = 1, . . . , s. If r = s and rk Ai = rk Bi for i = 1, . . . , r, then Σ is said to have the same rank as Σ . Repeating the proof of Theorem 2.4.3 we obtain that in every nonempty family of autoreduced sets of differential polynomials there exists an autoreduced set of lowest rank. If Q is an ideal of the ring K{y1 , . . . , ys }, then an autoreduced subset of Q of lowest rank is called a characteristic set of this ideal. (Clearly, the set of all autoreduced subsets of Q is not empty.) As in the proof of Proposition 2.4.4 we obtain that if Σ is a characteristic set of difference ideal Q of K{y1 , . . . , ys },

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271

then a σ-polynomials B ∈ Q is reduced with respect to the set Σ if and only if B = 0. Now we can complete the proof of Theorem 4.2.16 using the scheme of the proof of Theorem 3.3.16. Let P be the defining difference ideal of the σ-field extension L = Kη1 , . . . , ηs  and let Σ = {A1 , . . . , Ad } be a characteristic set of P . If p > 1, then for any r1 , . . . , rp ∈ N, let Ur1 ...rp = {u ∈ T Y | ordi u ≤ ri (1) (1) for i = 1, . . . , p and either u is not a transform of any uAi (i. e., u = τ uAi for (1)

any τ ∈ T ; i = 1, . . . , d), or for every τ ∈ T, A ∈ Σ such that u = τ uA , there (i) exists i ∈ {2, . . . , p} such that ordi (τ uA ) > ri }. If p = 1, we set Ur1 = {u ∈ (1) T Y | ord1 u ≤ r1 and u is not a transform of any uAi }. ¯r ...r = {u(η)|u ∈ Ur ...r } is a tranWe are going to show that the set U 1 p 1 p scendence basis of the field K({τ ηi | τ ∈ T (r1 , . . . , rp ), 1 ≤ j ≤ s} over K. ¯r ...r is algebraically independent First of all, let us prove that the set U 1 p over K. Let g be a polynomial in k variables (k ∈ N, k ≥ 1) such that g(u1 (η), . . . , uk (η)) = 0 for some elements u1 , . . . , uk ∈ Ur1 ...rp . Then the σpolynomial g¯ = g(u1 , . . . , uk ) is reduced with respect to Σ (if p > 1 and g (1) contains uj = τ uAi for some i, 1 ≤ i ≤ d, there exists i ∈ {2, . . . , p} such that (j)

(i)

ordi (τ uAi ) > ri ≥ ordi ug¯ ). Since g¯ ∈ P , we obtain that g¯ = 0, so the set ¯r ...r is algebraically independent over K. U 1 p Now, we are going to show that every element τ ηj (1 ≤ j ≤ s, τ ∈ ¯r ,...,r ). Let τ ηj ∈ ¯r ,...,r (if / U T (r1 , . . . , rp ) is algebraic over the field K(U 1 p 1 p ¯ / Ur1 ,...,rp whence τ yj τ ηj ∈ Ur1 ,...,rp , the statement is obvious). Then τ yj ∈ (1) is equal to some term of the form τ  uAi (τ  ∈ T , 1 ≤ i ≤ d) such that (k)

(1)

ordk (τ  uAi ) ≤ rk for all 1 < k ≤ p. Let us represent Ai as a polynomial in uAi : (1) e

(1) e−1

(1)

+ · · · + Ie , where I0 , I1 , . . . Ie do not contain uAi

Ai = I0 (uAi ) + I1 (uAi )

(1)

(therefore, all terms in these σ-polynomials are lower than uAi with respect to the order <1 ). Since Ai ∈ P , (1)

e

(1)

Ai (η) = I0 (η)(uAi )(η) + I1 (η)(uAi )(η)

e−1

+ · · · + Ie (η) = 0.

(4.2.8)

Since I0 is reduced with respect to Σ, I0 ∈ / P , so that I0 (η) = 0. If we apply τ  to the both sides of the equation (4.2.8), the resulting equation will show that (1) τ ηl |ordi τ¯ ≤ ri for the element τ  uAi (η) = τ ηj is algebraic over the field K({¯ (1)

i = 1, . . . , p; 1 ≤ l ≤ s, and τ¯yl <1 τ  uAi = τ yj }. Now, the induction on the set of terms T Y ordered by the relation <1 completes the proof of the fact that ¯r ...r (η) is a transcendence basis of the field K({τ ηi | τ ∈ T (r1 , . . . , rp ), 1 ≤ U 1 p j ≤ s} over K. (1) (1) Let Ur1 ...rp = {u ∈ T Y |ordi u ≤ ri for i = 1, . . . , p and u = τ uAj for any (2)

τ ∈ T ; j = 1, . . . , d} and Ur1 ...rp = {u ∈ ΘY |ordi u ≤ ri for i = 1, . . . , p and (1) there exists at least one pair i, j (1 ≤ i ≤ p, 1 ≤ j ≤ d) such that u = θuAj (i)

(2)

and ordi (θuAj ) > ri }. (If p = 1, then we set Ur1 ...rp = ∅). Clearly, Ur1 ...rp =
(2)  (2) (1) (1) Ur1 ...rp Ur1 ...rp and Ur1 ...rp Ur1 ...rp = ∅.

272

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

By Theorem 1.5.2, there exists a numerical polynomial ω1 (t1 , . . . , tp ) in p (1) variables t1 , . . . , tp such that ω1 (r1 , . . . , rp ) = Card Ur1 ...rp for all sufficiently p large (r1 . . . rp ) ∈ N and degti ω1 ≤ ni (i = 1, . . . , p). Furthermore, repeating the last part of the proof of Theorem 3.3.16, we obtain that there exists a numerical polynomial ω2 (t1 , . . . , tp ) in p variables t1 , . . . , tp such that (2) ω2 (r1 , . . . , rp ) = Card Ur1 ...rp for all sufficiently large (r1 . . . rp ) ∈ Np and degti ω2 ≤ ni (i = 1, . . . , p). Clearly, the polynomial φη = ω1 + ω2 satisfies conditions (i) and (ii) of Theorem 4.2.16. In order prove the last statement of the theorem, one just need to notice that if ζ = {ζ1 , . . . , ζq } is another system of σ-generators of L over K and φζ t1 , . . . , tp ) is the corresponding dimension polynomial, then there exist positive integers s1 , . . . , sp such that ηi ∈ K({τ ζk | τ ∈ T (r1 , . . . , rp ), 1 ≤ k ≤ q}) and ζk ∈ K({τ ηi | τ ∈ T (r1 , . . . , rp ), 1 ≤ i ≤ s}) for any i = 1, . . . , s and k = 1, . . . , q. This observation immediately implies the last statement of Theorem 4.2.16. 

Exercise 4.2.19 Let K be a difference field whose basic set σ is a union of two disjoint subsets, σ1 = {α1 , . . . , αm } and σ2 = {β1 , . . . , βn } (m ≥ 1, n ≥ 1). Let T , T1 and T2 be the free commutative semigroups generated by the sets σ, k m l1 any element τ = α1k1 . . . αm β1 . . . βnln ∈ T , let σ1 and σ 2 , respectively, and for m n ord1 τ = i=1 ki , ord2 τ = j=1 lj and ord τ = ord1 τ + ord2 τ . Furthermore, if r, s ∈ N, then Ti (r) will denote the set {τ ∈ Ti | ordi τ ≤ r} (i = 1, 2) and T (r, s) will denote the set {τ ∈ T | ord1 τ ≤ r, ord2 τ ≤ s}. Let L = Kη be a σ-field extension of K generated by a finite set η = {η1 , . . . , ηq } and for any r ∈ N let Lr = KT1 (r)ησ2 and Lr = KT2 (r)ησ1 (as before, if Φ ⊆ T and A ⊆ L, then φ(A) = {φ(a) | a ∈ A, φ ∈ Φ}). Prove that there exist two numerical polynomials, χ1 (t) and χ2 (t) with the following properties: (i) χ1 (r) = σ2 -trdegK Lr and χ2 (r) = σ1 -trdegK Lr for all sufficiently large r ∈ N. (ii) deg χ1 ≤ m, deg χ2 ≤ n, so the polynomials χ1 (t) and χ2 (t) can be written  

m n   t+i t+j and χ2 (t) = with integer coefficients ai ai bj as χ1 (t) = i j i=0 j=0 and bj . (iii) am = bn = σ-trdegK L. Hint : Apply the results of Exercises 3.6.15 Exercise 4.2.20 With the notation of the preceding exercise, assume that the elements of σ are automorphisms of K whose extensions to L are automorphisms of L. Let Γ1 and Γ2 denote the free commutative groups generated by the sets σ1 and σ2 , respectively, and for any r ∈ N let L∗r = KΓ1 (r)η∗σ2 and ∗ ∗ L∗∗ r = KΓ2 (r)ησ1 . Prove that there exist two numerical polynomials, χ1 (t) ∗ and χ2 (t) with the following properties:

4.2. DIMENSION POLYNOMIALS OF σ-FIELD EXTENSIONS

273

(i) χ∗1 (r) = σ2 -trdegK L∗r and χ∗2 (r) = σ1 -trdegK L∗∗ r for all sufficiently large r ∈ N. (ii) deg χ∗1 ≤ m, deg χ∗2 ≤ n, so the polynomials χ1 (t) and χ2 (t) can be written 2m a 2n b + o(tm ) and χ∗2 (t) = + o(tn ) where a = b = σ-trdegK L. as χ∗1 (t) = m! n! Formulate and prove an analog of this result and the result of Exercise 4.2.19 for the case when only elements of σ2 are automorphisms of K whose extensions are automorphisms of L. Theorems 4.2.16 and 4.2.17 allow us to assign a numerical polynomial to a prime inversive difference ideal in an algebra of difference or inversive difference polynomials. Definition 4.2.21 Let K be a difference field with a basic set σ = {α1 , . . . , αn }, K{y1 , . . . , ys } an algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and P a prime inversive difference ideal in K{y1 , . . . , ys }. Let η = (η1 , . . . , ηs ) be a generic zero of the ideal P . Then the σ-dimension polynomial φη|K (t) of the σ-field extension Kη1 , . . . , ηs /K is called the difference (or σ-) dimension polynomial of the ideal P . It is denoted by φP (t). If 

n  t+i , ai this polynomial is written in the canonical form (1.4.9), φP (t) = i i=0 then the coefficient an (which is equal to σ-trdegK Kη1 , . . . , ηs /K) is called the difference (or σ-) dimension of the ideal P ; it is denoted by σ-dim P . Furthermore, d = deg φP (t) and the coefficient ad are called the difference (or σ-) type and the typical difference (or the typical σ-) dimension of P . Similarly, if K is an inversive difference field with a basic set σ = {α1 ,. . . ,αn }, Q is a prime σ ∗ -ideal in an algebra of σ ∗ -polynomials K{y1 ,. . ., ys }∗ and ζ = (ζ1 , . . . , ζs ) is a generic zero of Q, then the σ ∗ -dimension polynomial ψζ|K (t) of the σ ∗ -field extension Kζ1 , . . . , ζs ∗ /K is called the σ ∗ -dimension polynomial of the ideal Q and denoted by ψQ (t). If this polynomial is written 

n  t+i , then the coefficient bn (which is in the form (4.2.6), ψQ (t) = bi 2i i i=0 equal to σ-trdegK Kη1 , . . . , ηs ∗ /K) is called the inversive difference (or σ ∗ -) dimension of Q; it is denoted by σ ∗ -dim Q. Furthermore, d = deg ψQ (t) and the coefficient bd are called the inversive difference (or σ ∗ -) type and the typical inversive difference (or the typical σ ∗ -) dimension of Q. Note that since every two generic zeros of a prime inversive difference ideal P in an algebra of σ- or σ ∗ - polynomials difference are equivalent (see Proposition 2.2.4), so the σ- and σ ∗ - dimension polynomials of P are well-defined. Proposition 4.2.22 Let K be a difference field with a basic set σ = {α1 ,. . . ,αn } and K{y1 , . . . , ys } an algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys over K. Let P and Q be prime inversive difference ideals in K{y1 , . . . , ys } such that P
Q. Then φQ (t) ≺ φP (t).

274

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

Similarly, if the σ-field K is inversive and P and Q are two prime σ ∗ -ideals in an algebra of σ ∗ -polynomials K{y1 , . . . , ys }∗ over K such that P
Q, then ψQ (t) ≺ ψP (t). PROOF. Let P and Q be prime inversive difference ideals of K{y1 , . . . , ys } such that P
Q. Furthermore, for every r ∈ N, let Ar denote the polynomial ring K[{τ yi | τ ∈ T (r), 1 ≤ i ≤ s}] and let Pr and Qr ,denote the prime ideals P ∩ Ar and Q ∩ Ar of this ring, respectively. Finally, let Fr and Gr denote the difference fields of quotients of the σ-rings Ar /Pr and Ar /Qr , respectively. Since P
Q, Pr
Qr for all sufficiently large r ∈ N. It follows that trdegK Fr < trdegK Gr for all sufficiently large r ∈ N (see Theorem 1.2.25(ii) ) hence φQ (t) ≺ φP (t). The statement about the inequality of σ ∗ -dimension polynomials can be obtained in the same way.  Exercise 4.2.23 Let K be a G-field of zero characteristic (where G is a finitely generated commutative group acting on the field K) and let L = Kη1 , . . . , ηs G be a G-field extension of K generated by a family η = (η1 , . . . , ηs ) (we use the terminology and assumptions considered in the last part of section 3.5). Prove that there exists a numerical polynomial θ(t) with the following properties:

(i) θ(t) = trdegK K(G(r)η1 · · · G(r)ηs ) for all sufficiently large r ∈ Z. (ii) deg θ(t) ≤ n where n = rank G, and the polynomial θ(t) can be written 

n  t+i where a0 , . . . , an ∈ Z and 2n |an . as θ(t) = ai i i=0 (iii) Let G0 be any free commutative subgroup of G such that rank G0 = an rank G. Then n is equal to the maximal number of of elements ζ1 , . . . , ζp ∈ L 2 such that the set {g(ζi ) | g ∈ G0 , 1 ≤ i ≤ p} is algebraically independent over K. (This number is called the G-transcendence degree of L over K and is denoted by G-trdegK L.)

4.3

Limit Degree

In this section we define and study an invariant of a difference field extension that play an important role in the theory of systems of algebraic difference equations. This invariant, called the limit degree of the extension, was introduced by R. Cohn [39] for ordinary case; the correspondent concept for partial difference field extensions was defined by P. Evanovich [61]. In what follows we present a modified version of the Evanovich’s approach. Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let T denote the free commutative multiplicative semigroup generated by the elements α1 , . . . , αn . In what follows we shall consider an order  on T such that τ = α1k1 . . . αnkn  τ  = α1l1 . . . αnln if and only if (kn , . . . , k1 ) ≤lex (ln , . . . , l1 ). Obviously, T is well-ordered with respect to .

4.3. LIMIT DEGREE

275

For any r1 , . . . , rn ∈ N, we set T
(r1 , . . . , rn ) = {τ ∈ T | τ  α1r1 . . . αnrn } and extend this notation to the case when the symbol ∞ replaces some ri (with the condition k < ∞ for any k ∈ N). Let L = KS be a difference (σ-) field extension of a difference field K generated by a finite set S. As usual, if T  ⊆ T and A ⊆ L, then T  (A) (or T  A ) denotes the set {τ (a) | a ∈ A, τ ∈ T  }. Furthermore, if σ  ⊂ σ, then K can be naturally treated as a difference field with a basic set σ  . A σ  -field extension of this σ  -field generated by a set S will be denoted by KSσ
. If (r1 , . . . , rn ) ∈ Nn and r1 ≥ 1, then the degree of the field extension K(T
(r1 ,. . ., rn )(S))/K(T
(r1 −1,. . ., rn )(S)) will be denoted by d(S; r1 ,. . ., rn ). (Obviously, d(S; r1 , . . . , rn ) is either a non-negative integer or ∞.) Lemma 4.3.1 With the above notation, d(S; r1 , . . . , rn ) ≥ d(S; r1 +p1 , . . . , rn + pn ) for every (p1 , . . . , pn ) ∈ Nn . PROOF. Clearly, it is sufficient to prove that d(S; r1 , . . . , rn ) ≥ d(S; r1 + 1, . . . , rn ) and d(S; r1 , . . . , rn ) ≥ d(S; r1 , . . . , ri−1 , ri + 1, ri+1 , . . . , rn ) for every i = 2, . . . , n. To prove the first inequality, notice that d(S; r1 , . . . , rn ) (r1 , . . . , rn )S) : α1 (K)(α1 T
(r1 − 1, . . . , rn
)S) ≥ K(α1 = α1 (K)(α1 T

T
(r1 , . . . , rn )S T (0, r2 , . . . , rn )S) : K(α1 T
(r1 − 1, . . . , rn )S T
(0, r2 , . . . , rn )S) = K(T
(r1 + 1, r2 , . . . , rn )S) : K(T
(r1 , . . . , rn )S) (we use the inequalities from Theorem 1.6.2(iv)), so that d(S; r1 , . . . , rn ) ≥ d(S; r1 + 1, . . . , rn ). Similarly, if 2 ≤ i ≤ n, then d(S; r1 , . . . , rn ) = αi (K)(αi T
(r
1 , . . . , rn )S) : αi (K)(αi T
(r1 − 1, r2 . . . , rn )S) ≥ K(αi T

(r1 , . . . , rn )S) T
(∞, . . . , ∞, 0, ri+1 , . . . , rn )S) : K(αi T
(r1−1, r2 , . . . , rn )S) T
(∞, . . . , ∞, 0, ri+1 , . . . rn )S) = d(S; r1 , . . . , ri−1 , ri + 1, ri+1 , . . . , rn ). This completes the proof.  The last lemma implies that if d(S; r1 , . . . , rn ) is finite for some (r1 , . . . , rn ) ∈ Nn , then min{d(S; r1 , . . . , rn ) | (r1 , . . . , rn ) ∈ Nn } is finite. In this case we denote this minimum value of d(S; r1 , . . . , rn ) by d(S). If d(S; r1 , . . . , rn ) = ∞ for all (r1 , . . . , rn ) ∈ Nn , we set d(S) = ∞. The following statement shows that d(S) does not depend on the system of generators S of L/K. Therefore, d(S) can be considered as a characteristic of this finitely generated difference extension; it is called the limit degree of L/K and denoted by ld(L/K). If a difference field extension L/K is not finitely generated, then its limit degree ld(L/K) is defined as the maximum of limit degrees of finitely generated difference subextensions N/K (K ⊆ N ⊆ L) if this maximum exists, or ∞ if it does not. As it follows from the multiplicative property of limit degree proved below, if a difference field extension L/K is finitely generated, then ld(L/K) is also the maximum of the limit degrees of the finitely generated subextensions of L/K (including L/K itself.) Lemma 4.3.2 Let K be a difference field with a basic set σ = {α1 , . . . , αn }, and let S and S  be two finite systems of generators of a difference (σ-) field extension L/K, that is, L = KS = KS  . Then d(S) = d(S  ).

276

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

PROOF. Let d = d(S), e = d(S  ), and let d(S; r1 , . . . , rn ) and d(S  ; r1 , . . . , rn ) denote the degrees K(T
(r1 , . . . , rn )(S)) : K(T
(r1 − 1, . . . , rn )S) and K(T
(r1 , . . . , rn )S  ) : K(T
(r1 − 1, r2 . . . , rn )S  ), respectively. It is easy to see that there exists a positive integer h such that K(S) ⊆ K(T
(h, . . . , h)S  ) and K(S  ) ⊆ K(T
(h, . . . , h)S). Since T
(k1 , . . . , kn )T
(l1 , . . . , ln ) = T
(k1 + l1 , . . . , kn + ln ) for any (k1 , . . . , kn ), (l1 , . . . , ln ) ∈ Nn , the last two inclusions imply that

and

K(T
(r1 , . . . , rn )S) ⊆ K(T
(r1 + h, . . . , rn + h)S  )

(4.3.1)

K(T
(r1 , . . . , rn )S  ) ⊆ K(T
(r1 + h, . . . , rn + h)S)

(4.3.2)

for every (r1 , r2 . . . , rn ) ∈ N . Assume, first, that d < ∞ and e < ∞. Then there exists m ∈ N such that d = K(T
(m, . . . , m)S) : K(T
(m − 1, m, . . . , m)S) and e = K(T
(m, . . . , m)S  ) : K(T
(m − 1, m, . . . , m)S  ) whence n

d = K(T
(m + p1 , . . . , m + pn )S) : K(T
(m + p1 − 1, m + p2 , . . . , m + pn )S) (4.3.3) and e = K(T
(m + p1 , . . . , m + pn )S) : K(T
(m + p1 − 1, m + p2 , . . . , m + pn )S) (4.3.4) for every (p1 , . . . , pn ) ∈ Nn . Therefore, for every integer k > h we have K(T
(m + k + h, . . . , m + k + h)S) : K(T
(m, . . . , m)S) = dn(k+h)

(4.3.5)

and K(T
(m + k, . . . , m + k)S  ) : K(T
(m + h, . . . , m + h)S  ) = en(k−h) . (4.3.6) Furthermore, the inclusions (4.3.1) and (4.3.2) imply that K(T
(m + k, . . . , m + k)S  ) ⊆ K(T
(m + k + h, . . . , m + k + h)S) and

K(T
(m, . . . , m)S) ⊆ K(T
(m + h, . . . , m + h)S  ).

(4.3.7) (4.3.8)

Combining (4.3.7) and (4.3.8) with the equalities (4.3.5) and (4.3.6) we obtain that (4.3.9) dn(k+h) ≥ en(k−h) for all k > h. Allowing k → ∞ in the last inequality we arrive at the inequality d ≥ e. Similarly we can obtain that e ≥ d whence e = d. Furthermore, inclusions (4.3.7) and (4.3.8) (and inclusions obtained from (4.3.7) and (4.3.8) by interchanging S and S  ) imply that if one of the values d(S), d(S  ) is finite, then the other value is also finite and d(S) = d(S  ). This completes the proof of the lemma.  The following proposition gives some relationships between the limit degree and difference transcendence degree.

4.3. LIMIT DEGREE

277

Proposition 4.3.3 Let K be a difference field with a basic set σ = {α1 , . . . , αn } and L a σ-field extension of K. Then (i) If σ-trdegK L > 0, then ld(L/K) = ∞. (ii) If the σ-field extension L/K is finitely generated and σ-trdegK L = 0, then ld(L/K) < ∞. PROOF. (i) Suppose that σ-trdegK L > 0, but ld(L/K) = d < ∞. Then L contains an element η which is σ-transcendental over K. Obviously, ld(Kη/K) ≤ ld(L/K) < ∞, so there exists (r1 , . . . , rn ) ∈ Nn such that K(T
(r1 , . . . , rn )η) : K(T
(r1 −1, r2 , . . . , rn )η) < ∞. In this case αr1 . . . αnrn (η) is algebraic over the field K(T
(r1 − 1, . . . , rn )η) that contradicts the fact that η is σ-transcendental over K. (ii) Let L = KS for some finite set S ⊆ L and let σ-trdegK L = 0. Assume, first that Card S = 1, S = {η}. The order  on T naturally determines a wellordering of the set of all terms τ y (τ ∈ T ) in the ring of difference polynomials K{y} in one difference indeterminate y. (Using the same symbol  for this well-ordering, we have τ y  τ  y if and only if τ  τ  .) Since η is σ-algebraic over K, there exists a difference polynomial P ∈ K{y} such that P (η) = 0 (as before, P (η) denotes the element of L obtained by replacing every τ y in P by τ η). Let u = τ y be the greatest term of P with respect to , and let τ = α1r1 . . . αnrn where r1 , . . . , rn ∈ N, r1 ≥ 1 (if r1 = 1, we can replace P by α1 P ). Then the element τ (η) is algebraic over K(T
(r1 − 1, . . . , rn )η) hence K(T
(r1 , . . . , rn )η) : K(T
(r1 − 1, . . . , rn )η) < ∞, hence ld(L/K) < ∞. Now let Card S > 1, S = {η1 , . . . , ηm }. As we have seen, for every i = 1, . . . , m, there exist ri1 , . . . , rin ∈ N such that K(T (ri1 , . . . , rin )ηi ) : K(T (ri1 − 1, . . . , rin )ηi ) < ∞. By Lemma 4.3.1, there exists a sufficiently large r ∈ N such that K(T
(r, . . . , r)ηi ) : K(T
(r − 1, r, . . . , r)ηi ) < ∞ for m m   i = 1, . . . , m. Then K( T
(r, . . . , r)ηi ) : K( T
(r − 1, r, . . . , r)ηi ) < ∞ i=1

(see Theorem 1.6.2(iii)), hence ld(L/K) < ∞.

i=1



Theorem 4.3.4 Let K be a difference field with a basic set σ, M a σ-field extension of K and L an intermediate σ-field of this extension. Then ld(M/K) = [ld(M/L)][ld(L/K)]. PROOF. If σ-trdegK M > 0, then ld(M/K) = ∞ (see Proposition 4.3.3) and at least one of the numbers σ-trdegL M , σ-trdegK L is positive, so the corresponding limit degree is ∞ and the statement of the theorem is true. Thus, we can assume that σ-trdegK M = 0, so that every element of M is σ-algebraic over K. Suppose, first, that L and M are finitely generated σ-field extensions of K. Let L = KX, M = KY  (X and Y are some finite sets), and let d = ld(L/K), e = ld(M/L), and f = ld(M/K).
(Because of our assumption, d, e, and f are finite). Obviously, M = KX Y  and there exist p1 , . . . , pn ∈ N such that

278

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS d = K(T
(r1 , . . . , rn )X) : K(T
(r1 − 1, r2 , . . . , rn )X), e = L(T
(r1 , . . . , rn )Y ) : L(T
(r1 − 1, r2 , . . . , rn )Y ),

and f = K(T
(r1 , . . . , rn )(X ∪ Y )) : K(T
(r1 − 1, r2 , . . . , rn )(X ∪ Y )) for all r1 , . . . , rn with (p1 , . . . , pn ) ≤P (r1 , . . . , rn ). (≤P denotes the product order on Nn ). It follows that for any (h1 , . . . , hn ) ∈ Nn , K(T
(r1 + h1 , . . . , rn + hn )X) : K(T
(r1 , . . . , rn )X) = dh1 +···+hn , L(T
(r1 + h1 , . . . , rn + hn )Y ) : L(T
(r1 , . . . , rn )Y ) = eh1 +···+hn , and K(T
(r1 + h1 , . . . , rn + hn )(X ∪ Y )) : K(T
(r1 , . . . , rn )(X ∪ Y )) = f h1 +···+hn . Let us show that there exists a finitely generated field (not necessarily a σ-field) subextension N/K of L/K such that N (T (p1 + 1, p2 , . . . , pn )Y ) : N (T (p1 , . . . , pn )Y ) = e.

(4.3.10)

Indeed, applying Theorem 1.6.2(v) to the sequence of field extensions K((T
(p1 , . . . , pn )Y ) ⊆ L((T
(p1 , . . . , pn )Y ) ⊆ L(T
(p1 +1, . . . , pn )Y ), we obtain that there exists a finite set W ⊆ L(T
(p1 , . . . , pn )Y ) such that K(W ∪ T
(p1 + 1, . . . , pn )Y ) : K(W ∪ T
(p1 , . . . , pn )Y ) = L(T
(p1 + 1, . . . , pn )Y ) : L(T
(p1 , . . . , pn )Y ) = e.  Since W ⊆ L((T
(p1 , . . . , pn )Y ) = K( τ (X) ∪ T
(p1 , . . . , pn )Y ), there τ ∈T
exists a finite set Λ ⊆ L such that W ⊆ K(Λ (T
(p1 , . . . , pn )Y ). By Theorem 1.6.2(iv), e = L(T
(p1 + 1, p2 . . . , pn )Y ) : L(T
(p1 , . . . , pn )Y )  ≤ K(Λ T
(p1 + 1, p2 , . . . , pn )Y ) : K(Λ ∪ T
(p1 , . . . , pn )Y )   T
(p1 , . . . , pn )Y ) = e. ≤ K(W T
(p1 + 1, p2 , . . . , pn )Y ) : K(W Setting N = K(Λ) we satisfy condition (4.3.10). Furthermore, since Λ ⊆ KX, there exist positive integers q1 , . . . , qn such that (p1 , . . . , pn ) ≤P (q1 , . . . , qn ) and N ⊆ K(T
(q1 , . . . , qn )X). Applying Theorem 1.6.2(iv) we obtain that K(T
(q1 +1, q2 , . . . , qn )X∪T
(p1 +1, p2 , . . . , pn )Y ) : K(T
(q1 +1, q2 , . . . , qn )X∪ T
(p1 , . . . , pn )Y ) ≤ N (T
(p1 + 1, p2 , . . . , pn )Y ) : N (T
(p1 , . . . , pn )Y ) = e. (4.3.11)

4.3. LIMIT DEGREE

279

and K(T
(q1 + 1, q2 , . . . , qn )X ∪ T
(p1 , . . . , pn )Y ) : K(T
(q1 , . . . , qn )X)∪ T
(p1 , . . . , pn )Y ) ≤ K(T
(q1 +1, q2 , . . . , qn )X) : K(T
(q1 , . . . , qn )X) ≤ d. (4.3.12) Combining (4.3.11) and (4.3.12) we arrive at the inequality K(T
(q1 + 1, q2 . . . , qn )X) ∪ T
(p1 + 1, p2 . . . , pn )Y ) : K(T
(q1 , . . . , qn )X)∪ T
(p1 , . . . , pn )Y ) ≤ de. Let U = T
(q1 , . . . , qn )X hence




T
(p1 , . . . , pn )Y . Since X




(4.3.13) Y ⊆ U , M = KU 

f ≤ K(U ∪α1 (U )) : K(U ) = K(T
(q1 +1, . . . , qn )X ∪T
(p1 +1, p2 , . . . , pn )Y ) : K(T
(q1 , . . . , qn )X ∪ T
(p1 , . . . , pn )Y ) ≤ de

(4.3.14)

In order to prove the opposite inequality f ≥ de, let us consider a transcendence basis B of the set T
(p1 , . . . , pn )Y over K(T
(p1 , . . . , pn )X) such n  that B = Bi where B1 is a transcendence basis of T
(p1 , . . . , pn )Y over i=1
K(T
(p1 , . . . , pn )X T
(0, p2 , . . . , pn )Y ) and for every i = 2, . . . , n, Bi is a
(p , . . . , p )X T (0, . . . , 0, pi , . . . , pn )Y ) transcendence basis of the field K(T
1 n

over K(T
(p1 , . . . , pn )X T
(0, . . . , 0, pi−1 , . . . , pn )Y ) contained in T
(0, . . . , 0, pi , . . . , pn )Y . Clearly, m = K(T
(p1 , . . . , pn )X ∪ T
(p1 , . . . , pn )Y ) : K(T (p1 , . . . , pn )X ∪ B) is finite, and if h ∈ N then every element of the set T
(p1 + h, p2 , . . . , pn )X is algebraic over K(T
(p1 , . . . , pn )X) (it follows from the fact that d < ∞). Therefore, for any h ∈ N, the set B is algebraically independent over K(T
(p1 + h, p2 , . . . , pn )X) and K(T
(p + h, p2 , . . . , pn )X ∪ B) : K(T
(p1 , . . . , pn )X ∪ B) = K(T
(p1 + h, p2 , . . . , pn )X) : K(T
(p1 , . . . , pn )X) = dh

(4.3.15)

Now, using the inclusions K(T
(p1 , . . . , pn )X ∪ B) ⊆ K(T
(p1 , . . . , pn )X ∪ T
(p1 , . . . , pn )Y ) ⊆ K(T
(p1 + h, p2 , . . . , pn )X ∪ T
(p1 , . . . , pn )Y ) and K(T
(p1 , . . . , pn )X) ∪ B) ⊆ K(T
(p1 + h, p2 , . . . , pn )X ∪ B) ⊆ K(T
(p1 + h, p2 , . . . , pn )X ∪ T
(p1 , . . . , pn )Y ) we obtain that [K(T
(p1 + h, p2 , . . . , pn )X ∪ T
(p1 , . . . , pn )Y ) : K(T
(p1 , . . . , pn )X)∪

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

280

T
(p1 , . . . , pn )Y )] · [K(T
(p1 + h, p2 , . . . , pn )X ∪ B) : K(T
(p1 , . . . , pn )X ∪ B)] ≥ K(T
(p1 + h, p2 , . . . , pn )X ∪ B) : K(T
(p1 , . . . , pn )X ∪ B). The last inequality, together with (4.3.15), implies that K(T
(p1 + h, p2 , . . . , pn )X ∪ T
(p1 , . . . , pn )Y ) : K(T
(p1 , . . . , pn )X∪ dh . m Furthermore, Theorem1.6.2(iv) yields the inequality T
(p1 , . . . , pn )Y ) ≥

(4.3.16)

K(T
(p1 +h, p2 , . . . , pn )X∪T
(p1 +h, p2 , . . . , pn )Y ) : K(T
(p1 +h, p2 , . . . , pn )Y ∪T
(p1 , . . . , pn )X) ≥ L(T
(p1 +h, p2 , . . . , pn )Y ) : L(T
(p1 , . . . , pn )Y ) = eh . (4.3.17) Combining inequalities (4.3.16) and (4.3.17) we obtain that f h = K(T
(p1 + h, p2 , . . . , pn )X ∪ T
(p1 + h, p2 , . . . , pn )Y ) : K(T
(p1 , . . . , pn ) × Y ∪ T
(p1 , . . . , pn )Y ) ≥

dh eh m

(4.3.18)

for all h ∈ N. Letting h → ∞ we see that the last inequality can hold only if de ≤ f . Thus, f = de for i = 1, . . . , n that completes the proof of the theorem for the case when L/K and M/K are finitely generated difference field extensions. Now suppose that K ⊆ L ⊆ M is any chain of difference fields and σ-trdegK M = 0. (As we have seen, the general case can be reduced to the case with this condition.) Let d, e and f be as before. Then the first part of the proof shows that if X and Y are finite subsets of L and M , respectively, then f ≥ ld(KX ∪ Y /K) = [ld(KX ∪ Y /KX)][ld(KXK)]

(4.3.19)

It is clear that KX(T
(r1 + 1, r2 , . . . , rn )Y ) : KX(T
(r1 , . . . , rn )Y ) ≥ L(T
(r1 + 1, r2 , . . . , rn )Y ) : L(T
(r1 , . . . , rn )Y ) for any r1 , . . . , rn whence ld(LY /L) ≤ ld(KX ∪ Y /KX). This inequality, together with (4.3.19), implies that f ≥ [ld(LY /L)][ld(KX/K)] . (4.3.20) If d < ∞ and e < ∞, one can choose X and Y such that the factors in the right-hand side of (4.3.20) are d and e. If either d or e is ∞, one can choose X or Y to make the corresponding factor arbitrarily large. In either case we obtain that f ≥ de. In order to prove the opposite inequality (and complete the proof of the theorem), it is sufficient to show that de ≥ f in the case d < ∞, e < ∞ (otherwise, de = ∞ ≥ f ). In other words, we should prove that if U is a finite subset of M , then (4.3.21) ld(KU /K) ≤ de . If M/L is finitely generated, then the first part of the proof shows that ld(LU /L) ≤ ld(M/L) = e. If M/L is not finitely generated, then we also

4.3. LIMIT DEGREE

281

have ld(LU /L) ≤ e by the definition of ld(M/L). It follows that there exist r1 , . . . , rn ∈ N such that L(T
(r1 + 1, r2 , . . . , rn )U ) : L(T
(r1 , . . . , rn )U ) ≤ e. Now, as in the first part of the proof, we obtain that there exists a finite set P ⊆ L such that K(P ∪ T
(r1 + 1, r2 , . . . , rn )U ) : K(P ∪ T
(r1 , . . . , rn )U ) ≤ e. Therefore, ld(KP ∪ U /KP ) ≤ e . (4.3.22) From the first part of the proof, if L/K is finitely generated, or by the definition of ld(L/K) otherwise, we have ld(KP /K) ≤ ld(L/K) = d .

(4.3.23)

Using the already proven statement for finitely generated extensions and (4.3.22), (4.3.23), we can evaluate ld(KP ∪ U /K) in two ways as follows: ld(KP ∪ U /K) = [ld(KP ∪ U /KP )][ld(KP /K)] ≤ de

(4.3.24)

ld(KP ∪ U /K) = [ld(KP ∪ U /KU )][ld(KU /K)].

(4.3.25)

and

Combining (4.3.24) and (4.3.25) we obtain that ld(KU /K) ≤ de. This completes the proof of the theorem.  Corollary 4.3.5 Let K be a difference field with a basic set σ = {α1 , . . . , αn } and K ∗ the inversive closure of K. Then ld(K ∗ /K) = 1. PROOF. If S is any finite subset of K ∗ , then there exist k1 , . . . , kn ∈ N such that α1k1 . . . αnkn (S) ⊆ K. It follows that for every (r1 , . . . , rn ) ∈ Nn with ri > ki (i = 1, . . . , n), K(T
(r1 , . . . , rn )S) : K(T
(r1 − 1, r2 , . . . , rn )S) = 1.  Therefore, ld(K ∗ /K) = 1. Corollary 4.3.6 Let K be a difference field with a basic set σ = {α1 , . . . , αn } and L a σ-field extension of K. Let L∗ be the inversive closures of L and K ∗ ⊆ L∗ the inversive closure of K. Then ld(L∗ /K) = ld(L∗ /K ∗ ) = ld(L/K). PROOF. Applying Theorem 4.3.4 to the chains of difference fields K ⊆ L ⊆ L∗ and K ⊆ K ∗ ⊆ L∗ and using the result of Corollary 4.3.5, we obtain that ld(L∗ /K) = [ld(L/K)][ld(L∗ /L)] = ld(L/K) and ld(L∗ /K) =  [ld(K ∗ /K)][ld(L∗ /K ∗ )] = ld(L∗ /K ∗ ). Using the separable degree of field extensions and its properties (see Theorem 1.6.23), one can define another characteristic of a difference field extension. Let K be a difference field with a basic set σ = {α1 , . . . , αn } and LS a σ-field extension of K generated by a finite set S. Setting e(S; r1 , . . . , rn ) = [K(T
(r1 , . . . , rn )(S)) : K(T
(r1 − 1, r2 . . . , rn )(S))]s for any r1 , . . . , rn ∈ N (e(S; r1 , . . . , rn ) is either a non-negative integer or ∞), we can repeat the proof

282

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

of Lemmas 4.3.1 and show that e(S; r1 , . . . , rn ) ≥ e(S; r1 + p1 , . . . , rn + pn ) for any n-tuples (r1 , . . . , rn ), (p1 , . . . , pn ) ∈ Nn . Now we can set e(S) = min{d(S; r1 , . . . , rn ) | (r1 , . . . , rn ) ∈ Nn } and use the arguments of the proof of Lemma 4.3.2 to show that e(S) = e(S  ) for any other finite set of σ-generators of L/K. It follows that e(S) can be considered as a characteristic of the difference field extension; it is called the reduced limit degree of L/K and denoted by rld(L/K). If a difference field extension L/K is not finitely generated, then its reduced limit degree rld(L/K) is defined as the maximum of reduced limit degrees of finitely generated difference subextensions N/K (K ⊆ N ⊆ L) if this maximum exists, or ∞ if it does not. Of course, in the case of difference fields of zero characteristic the concepts of reduced limit degree and limit degree are identical. Furthermore, it is easy to see that if Char K = p > 0, then d(S; r1 , . . . , rn ) (considered in the definition of the limit degree of L = KS (Card S < ∞) over K) is a multiple of e(S; r1 , . . . , rn ) by a (non-negative integer) power of p and the same can be said about d(S) and e(S). Therefore, in this case ld(L/K) is a multiple of rld(L/K) by a power of p. Using the arguments of the proof of Theorem 4.3.4 one can easily obtain the following multiplicative property of the reduced limit degree and its consequence. Theorem 4.3.7 Let K be a difference field with a basic set σ, M a σ-field extension of K and L an intermediate σ-field of this extension. Then rld(M/K) = [rld(M/L)][rld(L/K)].  Corollary 4.3.8 Let K be a difference field with a basic set σ and L a σ-field extension of K. Furthermore, let L∗ be the inversive closures of L and K ∗ ⊆ L∗ the inversive closure of K. Then rld(L∗ /K) = rld(L∗ /K ∗ ) = rld(L/K).



Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let ∆ = {αi1 , . . . , αik } (1 ≤ k ≤ n) be a subset of σ whose elements are automorphisms of K. If we replace each αj ∈ ∆ by αj−1 and do not change elements of σ \ ∆, then the resulting set will be denoted by σ∆ . Obviously, K can be treated as a difference field with the basic set σ∆ . If L is a σ-field extension of K such that the extensions of αj ∈ ∆ are automorphisms of L, then L can be also treated as a σ∆ -field extension of the σ∆ -field K. The limit degree and reduced limit degree of this extension are called, respectively, the σ∆ -limit degree and reduced σ∆ -limit degree of L/K. They are denoted by σ∆ -ld(L/K) and σ∆ -rld(L/K), respectively. Exercise 4.3.9 With the above notation, prove that if M is a σ-field extension of L such that the extensions of αj ∈ ∆ are automorphisms of M , then [σ∆ -ld(M/K) = [σ∆ -ld(L/K)][σ∆ -ld(M/L)] and [σ∆ -rld(M/K) = [σ∆ -rld(L/K)][σ∆ -rld(M/L)].

4.3. LIMIT DEGREE

283

The following example illustrates that the definition of the limit degree of a partial difference field extension essentially depends on the order of translations in the basic set. Example 4.3.10 Let K be a difference field with a basic set σ = {α1 , α2 } and let K{y} be the ring of σ-polynomials in one difference indeterminate y over K. In what follows we consider the set of terms T y = {τ y | τ ∈ T } together with its two rankings ≺1 and ≺2 such that α1k1 α2k2 y ≺i α1l1 α2l2 y if and only if ki < li or ki = li and k2−i < l2−i (i = 1, 2). Let f = (α1 y)2 + α1 y + α2 y + y ∈ K{y}. Then u = α1 y and v = α2 y are leaders of f with respect to the rankings ≺1 and ≺2 , respectively. Since f is linear with respect to v, this σ-polynomial is irreducible and so is any σ-polynomial τ f (τ ∈ T ). Furthermore, it is easy to see that the ideal [f ] is prime. Indeed, suppose that g1 g2 ∈ [f ] where g1 , g2 ∈ K{y} \ [f ]. Considering the set of terms T y together with the ranking ≺2 and applying the reduction theorem (Theorem 2.4.1), we obtain that there exist nonzero σ-polynomials g1 , g2 ∈ K{y} such that gi is reduced with respect to f and gi − gi ∈ [f ] (i = 1, 2). It follows that no term of the form α1r α2s y with s ≥ 1 appears in g1 or g2 , hence such a term cannot appear in g1 g2 . On the other hand, g1 g2 ∈ [f ] hence degv g1 g2 ≥ 1 (see Theorem 2.4.15). This contradiction shows that the difference ideal [f ] is prime. Let L = Kη be a σ-field extension of K generated by a single element η with the defining equation (α1 η)2 + α1 η + α2 η + η = 0. (In other words, [f ] is the defining ideal of the generator η over K.) In what follows, if S is a set of elements in a σ-field extension of K, then KS{αi } will denote the ordinary difference field extension of K generated by the set S when K is treated as an ordinary difference field with the basic set {αi } (i = 1, 2). Let us compute the limit degrees of L over K with respect to two orders of the set T that correspond to the rankings ≺1 and ≺2 of T y (we shall use the same symbols for these orders): α1k1 α2k2 ≺i α1l1 α2l2 y if and only if ki < li or ki = li and k2−i < l2−i (i = 1, 2). For any k, l ∈ N, let Ti (k, l) = {τ ∈ T | τ ≺i α1k α2l } and denote the limit degree of the extension L/K with respect to ≺i by ldi (L/K) (i = 1, 2). Then there exist r, s ∈ N+ such that ld1 (L/K) = K(T2 (r, s)η) : K(T2 (r − 1, s)η) and ld2 (L/K) = K(T1 (r, s)η) : K(T1 (r, s − 1)η). Since f (η) = (α1 η)2 + α1 η + α2 η + η = 0, α2 η ∈ Kη{α1 } and α1r α2s η ∈ Kη{α1 } . Therefore, ld1 (L/K) = 1. Let us show that ld2 (L/K) = 2. First we apply the endomorphism α1r−1 α2s to the left-hand side of the defining equation for η and obtain that α1r α2s η satisfy a polynomial equation of second degree over the field K(T1 (r, s − 1)η). Now it remains to show that α1r α2s η does not lie in K(T1 (r, s − 1)η). Suppose that α1r α2s η ∈ K(T1 (r, s − 1)η) and let w denote the term α1r α2s y ∈ T y. Then there exists a σ-polynomial h = Aw + B ∈ K{y} such that A, B ∈ K[T (r, s − 1)y], A ∈ / [f ] (that is, A(η) = 0), and h(η) = 0, so that h ∈ [f ]. Let us apply the reduction process described in the proof of

284

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

the reduction theorem (Theorem 2.4.1) to h. We obtain a σ-polynomial h1 ∈ [f ] which is reduced with respect to f and can be written as h1 = A1 w+B1 ∈ K{y} / [f ]. On the other hand, Theorem 2.4.15 where A1 , B1 ∈ K[T (r, s−1)y] and A1 ∈ shows that [f ] contains no nonzero σ-polynomial reduced with respect to f . / K(T1 (r, s − 1)η) whence ld2 (L/K) = 2. Thus, α1r α2s η ∈ In what follows we consider some peculiar results on limit degree of ordinary difference field extensions. Let K be an ordinary difference field with a basic set σ = {α} and L = KS a σ-field extension of K generated by a finite set S. Let S0 = S and for every k = 1, 2, . . . , let Sk denote the set {αi (s) | s ∈ S, 0 ≤ i ≤ k} and dk = K(Sk ) : K(Sk−1 ).
Then Lemma 4.3.1
shows that dk = α(K)(α(Sk ) ) : α(K)(α(Sk−1 ) ) ≥ K(S α(Sk ) ) : K(S α(Sk−1 ) ) = dk+1 for k = 1, 2, . . . . Let d(S) = min{dk | k = 1, 2, . . . } if some dk is finite, or d(S) = ∞ if all dk are infinite. By Lemma 4.3.2, d(S) does not depend on the system of difference generators S of L/K; as before, this invariant of the extension L/K is called the limit degree of the extension and denoted by ld(L/K). As in the case Card σ > 1, if L/K is not finitely generated, its limit degree ld(L/K) is defined to be the maximum of the limit degrees of all finitely generated difference subextensions of L/K, if this maximum exists, or ∞ if it does not. Of course, limit degree of ordinary difference field extensions has the properties listed in Proposition 4.3.3, the multiplicative property (see Theorem 4.3.4), and the properties stated in Corollaries 4.3.5 and 4.3.6. Also, the use of the separable degree of field extensions instead of the degree, leads to the concept of reduced limit degree of an ordinary field extension L/K and its properties formulated in Theorem 4.3.7 and Corollary 4.3.8. If K is an inversive difference field with a basic set σ = {α}, then K can be also treated as a difference field with the basic set σ  = {α−1 } called the inverse difference field of K. It is denoted by K  . Let L be a σ-field extension of K and L the inverse difference field of L (so that L is a σ  -field extension of K  ). The inverse limit degree of L over K is defined to be ld L /K  (it is denoted by ild L/K), and the inverse reduced limit degree of L over K is defined to be rld L /K  (it is denoted by irld L/K). Exercises 4.3.11 Let K be an ordinary difference field with a basic set σ. 1. Prove that if M is a σ-field extension of K and L an intermediate σ-field of M/K, then ild(M/K) = [ild(M/L)][ild(L/K)] and irld(M/K) = [irld(M/L)][irld(L/K)] (cf. Exercise 4.3.9). 2. Let L be a σ-field extension of K, L∗ the inversive closures of L and K ∗ ⊆ ∗ L the inversive closure of K. Prove that rld(L∗ /K) = rld(L∗ /K ∗ ) = rld(L/K). Proposition 4.3.12 Let K be an ordinary difference field with a basic set σ = {α}. (i) If L is a finitely generated σ-field extension of K, then ld(L/K) = 1 if and only if L = K(V ) for some finite set V ⊆ L. (ii) The following two statements are equivalent:

4.3. LIMIT DEGREE

285

(a) L/K is a finitely generated σ-field extension, L is algebraic over K, and ld(L/K) = 1. (b) L : K is finite. (iii) If B is a set of elements in some σ-overfield of L, then ld(LB/KB) ≤ ld(L/K) and ld(LB/L) ≤ ld(KB/K). PROOF. (i) Let L = KS and ld(L/K) = 1. Then there exists a non
p+h
p+h+1 negative integer p such that K( i=0 αi (S)) : K( i=0 αi (S)) = 1, that is,
p+h i
p+h+1 K(
i=0 αi (S)) = K( i=0 α (S)) for all h ∈ N. It follows that L = KS =
∞ p i (S)) = K( αi (S)), so that L is generated over K by the finite K( i=0 α i=0
p i set V = i=0 α (S).
Conversely, if L = K(V ) for some finite set V , then K(V α(V )) : K(V ) = 1, hence ld(L/K) = 1. (ii) It is easy to see that statement (b) implies (a). Conversely, if a σ-field extension L/K is finitely generated, L is algebraic over K and ld(L/K) = 1, then the first part of the theorem shows that L = K(V ) for some finite set V . Since L/K is algebraic, L : K < ∞ (see Theorem 1.6.4(ii)). The last statement of the proposition is a direct consequence of the definition of limit degree and Theorem 1.6.2(iv).  Proposition 4.3.13 Let K be an ordinary difference field with a basic set σ = {α} and let L be an algebraic σ-field extension of K. Then ld(L/K) = ild(L/K) and rld(L/K) = irld(L/K). PROOF. It is easy to see that if L∗ and K ∗ ⊆ L∗ are the inversive closures of L and K, respectively, then the field extension L∗ /K ∗ is algebraic. Since ld(L∗ /K ∗ ) = ld(L/K), rld(L∗ /K ∗ ) = rld(L/K) (see Corollaries 4.3.6, 4.3.8) and similar equalities hold for ild(L/K) and irld(L/K) (see Exercises 4.3.11), we can assume from the very beginning that the difference fields K and L are inversive. Furthermore, without loss of generality we can assume that L is a finitely generated σ-field extension of K, L = KS for some finite set S. Let K  denote the inverse difference field of K, and let L be the inverse difference field of the inversive closure of L. Then L is the inversive closure of K  S, so ld(L /K  ) = ld(K  S/K  ). Let d = ld(L/K) and d = ild(L/K) = ld(L /K  ) = d(K  S/K  ). By the definition of the limit
degree, there exists p ∈ N such that for
p+h−1 p+h every h = 1, 2, . . . we have d = K( i=0 αi (S)) : K( i=0 αi (S)). Therefore,
p
p+h
p
p+h K( i=0 αi (S)) : K( i=0 αi (S)) = dh and K( i=0 α−i (S)) : K( i=0 α−i (S)) = (d )h .
p
p Setting e = K( i=0 αi (S)) : K and e = K( i=0 α−i (S)) : K (as degrees of finitely generated algebraic field extensions, e and e are finite), we obtain that
p+h
p+h for every positive integer h, K( i=0 αi (S)) : K = edh and K( i=0 α−i (S)) : K = e (d )h . Since αp+h is an automorphism of K and an isomorphism of
p+h
p+h K( i=0 α−i (S)) onto K( i=0 αi (S)), the last two equalities imply that edh = e (d )h . Letting h → ∞ in the last equation we obtain that d = d . The proof of the equality of the reduced limit degrees is similar. 

286

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

Proposition 4.3.14 Let K be an ordinary inversive difference field with a basic set σ = {α} and let L be an algebraic difference field extension of K such that ld(L/K) = 1. Then L is inversive. PROOF. Clearly, it is sufficient to consider the case that L is a finitely generated σ-field extension of K. Then L : K = α(L) : α(K) = α(L) : K < ∞. It follows that α(L) = L, so the σ-field L is inversive.  Exercise 4.3.15 Let K be an ordinary difference field with a basic set σ and let L be a finitely generated σ-field extension of K such that L/K is algebraic. Prove that rld(L/K) = 1 if and only if [L : K]s < ∞. The following example shows that there are proper algebraic ordinary difference field extensions L/K such that Char K = 0 and ld(L/K) = 1. Example 4.3.16 Let us consider the field of rational numbers Q as an ordinary difference field whose basic set σ consists of the identity automorphism α. If one adjoins to Q an element i such that i2 = −1, then the resulting field L = Q(i) together with the identity automorphism (also denoted by α) can be treated as a σ-field extension of Q. Clearly, L/Q is an algebraic finitely generated σ-field extension such that ld(L/Q) = 1 ({i} is the set of σ-geneartors of L/Q and for
h
h+1 every positive integer h, Q( k=0 αk (i)) : Q( k=0 αk (i)) = 1). Definition 4.3.17 Let K be an ordinary difference field and L a difference field extension of K. Then the core LK of L over K is defined to be the set of elements a ∈ L algebraic and separable over K and such that ld(Ka/K) = 1. It follows from Theorem 4.3.4 and its corollary that the core LK is an inversive intermediate σ-field of the extension L/K such that ld(LK /K) = 1. Furthermore, Example 4.3.16 shows that LK need not to be K. Also, if Char K = 0 or the field extension L/K is separable, Proposition 4.3.12 implies that L = LK if and only if L : K is finite. Exercise 4.3.18 With the above notation, show that if L is a finitely generated σ-field extension of K, then the extension L/LK is purely inseparable if and only if [L : K]s < ∞. Theorem 4.3.19 Let K be an ordinary inversive difference field with a basic set σ = {α}. Let L be an algebraic difference field extension of K and LK the core of L over K. Then a ∈ LK if and only if a ∈ Kα(a). In particular, the σ-field LK is inversive. PROOF. Let a ∈ LK . Then Ka : K < ∞ hence there exists r ∈ N such that Ka = K(a, α(a), . . . , αr (a)). Since α is an automorphism of the field K, K(α(a), α2 (a), . . . , αr+1 (a)) : K = K(a, α(a), . . . , αr (a)) : K = Ka : K. It follows that K(α(a), α2 (a), . . . , αr+1 (a)) = Ka whence a ∈ Kα(a).

4.3. LIMIT DEGREE

287

Conversely, suppose that a ∈ Kα(a). Then there exists n ∈ N such that a ∈ K(α(a), . . . , αn+1 (a)). Therefore, K(a, α(a), . . . , αn+1 (a)) = K(α(a), . . . , αn+1 (a)) .

(4.3.26)

Since α is an automorphism of the field K, K(a, α(a), . . . , αn (a)) : K = K(α(a), . . . , αn+1 (a)) : K. Furthermore, equality (4.3.26) shows that K(α(a), . . . , αn+1 (a)) ⊇ K(a, α(a), . . . , αn (a)). Therefore, K(α(a), . . . , αn+1 (a)) = K(a, α(a), . . . , αn (a))

(4.3.27)

whence K(a, α(a), . . . , αn (a)) = K(a, α(a), . . . , αn (a), αn+1 (a)). It follows that ld (Ka / K) = 1 and a ∈ LK .



Theorem 4.3.20 Let K and L be as in Theorem 4.3.19, Char K = 0, and let L = KS where S is a finite subset of L. Then ∞ # Kαn (S) . (4.3.28) LK = n=0

PROOF, Let a ∈ LK . Repeatedly applying Theorem 4.3.19 we obtain that ∞ # Kαn (a). Since a ∈ KS, a ∈ Kαn (a) for n = 1, 2, . . . , so that a ∈ n=0

Kαn (a) ⊆ Kαn (S) for all n ∈ N whence a∈

∞ # n=0

Kαn (S). Thus, LK ⊆

∞ #

∞ #

Kαn (a) ⊆

n=0

∞ #

Kαn (S) and

n=0

Kαn (S).

n=0

Now let us prove the opposite inclusion. Without loss of generality we can assume that the set S consists of a single element b (that is, L = Kb) and K(b, α(b)) : K = d where d denotes the limit degree of L over K. Indeed, let ld(L/K) = K(S, α(S), . . . , αm+1 (S)) : K(S, α(S), . . . , αm (S)) for some m ∈ N, m ≥ 1. By the theorem on a primitive element, there exists an element b ∈ K(S, α(S), . . . , αm (S)) such that K(b) = K(S, α(S), . . . , αm (S)). It is easy to see that Kb = KS = L, K(b, α(b)) = K(S, α(S), . . . , αm+1 (S)) ∞ ∞ # # n Kα (b) = Kαn (S). (Clearly, (hence K(b, α(b)) : K(b) = d), and n=0

n=0

Kαn (b) = Kαn (S) for n = 1, 2, . . . .) Let s denote the degree of b over K, that is, the degree of the minimal polynomial of b in the polynomial ring K[X]. Then every element αi (b) (i ∈ N) is algebraic of degree s over K. Furthermore, for any t ∈ N, t ≥ 1, K(αi (b), αi+1 (b), . . . , αi+t (b)) : K(αi (b), αi+1 (b), . . . , αi+t−1 (b)) = K(b, α(b), . . . , αt (b)) : K(b, α(b), . . . , αt−1 (b)) = d. Therefore, K(αi (b), αi+1 (b), . . . , αi+t (b)) : K = sdt and the set {αi (b)k0 αi+1 (b)k1 . . . αi+t (b)kt | 0 ≤ k0 ≤ s, 0 ≤ kν ≤ d − 1 for ν = 1, . . . , t} is a basis of the vector K-space K(αi (b), αi+1 (b), . . . , αi+t (b)).

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

288

Now, suppose that a ∈

∞ #

Kαn (b). Since a ∈ Kb, there exists r ∈ N

n=0

such that a ∈ K(b, α(b), . . . , αr (b)), so that a is a finite linear combination with coefficients from K of elements of the form bk0 α(b)k1 . . . αr (b)kr where 0 ≤ k0 ≤ s and 0 ≤ kν ≤ d − 1 for ν = 1, . . . , r. At the same time, if p ∈ N, p ≥ r + 1, then a ∈ Kαp (b), hence a ∈ K(αp (b), αp+1 (b), . . . , αp+q (b)) for some non-negative integer q. We assume that q is the smallest such a number. The last inclusion shows that a can be represented as a linear combination with coefficients from K of elements of the form αp (b)k0 αp+1 (b)k1 . . . αp+q (b)kq where 0 ≤ k0 ≤ s and 0 ≤ kν ≤ d − 1 for ν = 1, . . . , q. Comparing the two representations of a we obtain that if q ≥ 1, then αp+q (b) is a root of a polynomial of degree at most d − 1 with coefficients from the field K  = K(b, α(b), . . . , αp+q−1 (b)). This contradicts the fact that K  (αp+q (b)) : K  = d, that is, the minimal polynomial of αp+q (b) over K  has degree d. Therefore, ∞ # K(αj (b)) which q = 0 and a ∈ K(αp (b)). We arrive at the inclusion a ∈ implies that α(a) ∈

∞ #

K(αj (b)), α2 (a) ∈

j=p+2

∞ #

j=p+1

K(αj (b)), . . . .

j=p+3

Since K(αp+s+1 (b)) : K = s and a, α(a), . . . , αs (a) ∈

∞ #

K(αj (b)) ⊆

j=p+s+1

K(αp+s+1 (b)), elements a, α(a), . . . , αs (a) are linearly dependent over K. It follows that there exists the smallest positive integer k such that the elements a, α(a), . . . , αk (a) are linearly dependent over K. Then αk (a) is a linear combination of a, α(a), . . . , αk−1 (a) over K hence K(a, α(a), . . . , αk (a)) = K(a, α(a), . . . , αk−1 (a)). It follows that ld(Ka/K) = 1 and a ∈ LK . This completes the proof of the theorem.  Example 4.3.21 Let K = Q(x, y0 , y1 , y−1 , y2 , y−2 , . . . ) be the field of rational fractions in a denumerable set of indeterminates x, y0 , y1 , y−1 , y2 , y−2 , . . . over Q. Let us consider K as an inversive ordinary difference field with a basic set σ = {α} where α acts on Q as the identity mapping, = x, and α(yr ) = yr+1 √ α(x) √ for every r ∈ Z. Let u and ti (i ∈ N) denote x and yi , respectively, and let L = K(u, t0 , t1 , . . . ). (Thus, the field L is obtained by adjoining to K a root of the polynomial X 2 − x ∈ K[X] and the denumerable set of roots of the polynomials X 2 − yi , i ∈ N.) The field L can be naturally considered as a difference field extension of K where α(u) = u and α(ti ) = ti+1 for i = 0, 1, 2, . . . . It is easy to see that the set S = {u, t0 } generates this σ-field extension, L = KS = Ku + t0  and ∞ # Kαn (S) = Kαn (S) = Ku + tn  for n = 0, 1, 2, . . . . In this case LK = ∞ #

n=0

Ku + tn  = K(u). (With the notation of the proof of the last theorem,

n=0

b = u + t0 , s = 4, and d = 2.)

4.3. LIMIT DEGREE

289

Theorem 4.3.22 Let K(CharK = 0) and L = KS (Card S < ∞) be as in Theorem 4.3.19. Then ∞ # Kαn (S) = K, that is, LK = K. (i) If ld(L/K) = K(S) : K, then n=0

(ii) L = LK if and only if L : K is finite. PROOF. (i) As in the proof of Theorem 4.3.20, without loss of generality we can assume that the set S consists of a single element b such that K(b) : K = K(b, α(b)) : K(b) = d where d = ld (L/K). Furthermore, the arguments of the same proof show that the set B = {αi1 (b)k1 . . . αim (b)km | m ∈ N, 0 ≤ i1 < i2 < · · · < im and 0 ≤ kν ≤ d − 1 for ν = 1, . . . , m} is a basis of the vector K-space L = Kb, while for every r ∈ N, its subset Br = {αi1 (b)k1 . . . αim (b)km | m ≥ 0; r ≤ i1 < i2 < · · · < im ; and 0 ≤ kν ≤ d−1 for ν = 1, . . . , m} is a basis of the vector K-space Kαr (b). ∞ # Kαn (S) then a ∈ Kb hence a = λ1 b1 + · · · + λp bp for some If a ∈ n=0

λ1 , . . . , λp ∈ K and b1 , . . . , bp ∈ B. Let αs (b) be the highest transform of b that appears in b1 , . . . , bp . Since a ∈ Kαs+1 (b), a can be written as a = µ1 b1 + · · · + µq bq for some µ1, . . . , µq ∈ K and b1 , . . . , bq ∈ Bs+1 . We arrive at the equality λ1 b1 + · · · + λp bp = µ1 b1 + · · · + µq bq that can hold only if the both expressions ∞ # Kαn (S) = K. in its sides belong to K. Therefore, a ∈ K, so that n=0

(ii) If L : K is finite, then ld(L/K) = 1 (see Proposition 4.3.12), hence ld(Ka/K) = 1 for every a ∈ L, so that L = LK . Conversely, if L = LK and S = {a1 , . . . , am }, then ld(Ka1 /K) = 1 and ld(Ka1 , . . . , ai /Ka1 , . . . , ai−1 ) = 1 for 1 < i ≤ m (by Proposition 4.3.12 (iii), 1 ≤ ld(Ka1 , . . . , ai /Ka1 , . . . , ai−1 ) ≤ ld(Kai /K) = 1). Applying Theorem 4.3.4 we obtain that ld(L/K) = 1, and Proposition 4.3.12 shows that L : K is finite.  Exercise 4.3.23 Let K be an ordinary difference field of arbitrary characteristic and let L be a finitely generated difference field extension of K. Prove that L is purely inseparable over LK if and only if [L : K]s is finite. Theorem 4.3.24 Let K be an ordinary difference field with a basic set σ = {α} and let L be a separably algebraic σ-field extension of K. Furthermore, let K ∗ denote the inversive closure of K and let M be a σ-overfield of K ∗ L contained in the inversive closure L∗ of L. Then MK ∗ is the inversive closure of LK . PROOF. If a ∈ MK ∗ ⊆ L∗ , then there exists r ∈ N such that αr (a) ∈ L. Furthermore, both a and αr (a) are separably algebraic over K (it follows from

290

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

the definition of a core). By Proposition 4.3.14, the σ-field K ∗ a is inversive, so we can apply Corollary 4.3.6 and obtain that ld(Kαr (a)/K) = ld( (Kαr (a))∗ /K ∗ ) = ld(K ∗ a/K ∗ ) = 1, that is, αr (a) ∈ LK and a is in the inversive closure of LK . Furthermore, by Theorem 4.3.18, the σ-field MK ∗ is inversive and contains LK (repeating the above reasonings one can easily obtain that if a ∈ LK , then ld(K ∗ a/K ∗ ) = 1). Therefore, MK ∗ contains the inversive closure of LK , and hence coincides with this closure.  The following result provides an important property of finitely generated algebraic difference field extensions of limit degree one. Theorem 4.3.25 Let K be an inversive ordinary difference field with a basic set σ = {α}, let L be a σ-field extension of K such that L = Kη1 , . . . , ηm  where elements η1 , . . . , ηm are σ-algebraically independent over K (and therefore σ-trdegK L = m). Let M be a σ-overfield of L such that the field extension M/L is algebraic and ld(M/L) = 1. Then M is generated by adjoining to L a set of elements which are algebraic over K. PROOF. Without loss of generality we may assume that the field K is algebraically closed in L. With this assumption we should show that M = L. We proceed by induction on m. Let m = 1, so that L = Kη where η is σ-transcendental over K. Let u be an element in M . Since ld(M/L) = 1, there is r ∈ N such that αr+k (u) ∈ K(u, . . . , αt (u)) for any k ∈ N. Let us choose k so large that the set S of transforms αj (u) occurring in the polynomials Irr(αi (u), K), i = 1, . . . , r and the set T of transforms αj (u) occurring in Irr(αr+k (u), K) are disjoint. (It is possible, since Irr(αj (u), K) can be obtained by applying αj to each coefficient of Irr(u, K). ) Let the rational expression for αr+k (u) in terms of u, . . . , αr (u) and transforms of η be arranged as a rational function in members of T whose coefficients are rational combinations of other transforms αi (η) and u, . . . , αr (u) with one coefficient unity. Let S  be the set of transforms αi (u) appearing in these coefficients, and let U = S ∪ S  . Then U ∩ T = ∅. Let us adjoin to the field K new algebraically independent sets U (1) , U (2) , . . . , such that Card U (i) = Card U for every i = 1, 2, . . . . Taking bijections U → U (i) (where the image of an element a ∈ U is denoted by ai ) and the identical mapping T → T we can extend them to a K-isomorphism θi : K(U ∪ T ) → K(U (i) ∪ T ) which, in turn, can be extended to a field isomorphism θ¯i : K(U ∪ T )(u, . . . , αr (u), αr+k (u)) → K(U (i) ∪ T )(ui , . . . , αr (ui ), αr+k (ui )). If the rational expression for αr+k (u) contains a coefficient which is not algebraic over K, then, since the sets U (i) are algebraically independent over K, this coefficient has distinct images under the isomorphisms θ¯i . Then the αj (ui ) are all distinct. Since they are the zeros of Irr(αr+k (u), K) which is unaltered by the isomorphisms, this is impossible. Therefore, every coefficient is algebraic

4.3. LIMIT DEGREE

291

over K. But these coefficients lie in M , and since K is algebraically closed in M , the coefficients must be in K. Thus, αr+k (u) ∈ L, hence u ∈ L. It follows that M = L. To perform the induction step we set L = Kη1 , . . . , ηm  and assume that our statement is true if the number of σ-generators of L over K which form a σ-transcendence basis of L/K is less than m. Then M is generated by adjoining to L a set Σ of elements algerbaic over Kη1 . Since Kη1 Σ and L are linearly disjoint over Kη1 , ld(Kη1 Σ/K) = 1. Therefore, Kη1 Σ is generated by adjoining to L elements algebraic over K. This completes the proof of the theorem.  As we have mentioned, the first generalization of the concept of limit degree to the case of partial difference field extensions is due to P. Evanovich [61]. Considering a difference field K with a basic set σ = {α1 , . . . , αn } and its finitely generated σ-field extension M = KS, P. Evanovich inductively defined
a characteristic ldn (M/K) of this extension as an element of the set N {∞} satisfying the following conditions (ld1) - (ld5). (ld1) If M = LS for a finite set S ⊆ M , then there exists a finitely generated σ-overfield K  of K contained in L such that ldn (M/L) = ldn (K  S/K  ). (ld2) If S ⊆ M , then ldn (LS/KS) ≤ ldn (L/K) and ldn (LS/L) ≤ ldn (KS/K). Equality will hold in both if S is σ-algebraically independent over L. (ld3) If there is a σ-isomorphism φ of L onto a σ-field L and K  is a σ-subfield of L such that φ(K) = K  , then ldn (L/K) = ldn (L /K  ). (ld4) If the σ-field extension L/K is finitely generated, then σ-trdegk L = 0 if and only if ldn (L/K) < ∞. (ld5) ldn (M/K) = ldn (M/L) · ldn (L/K). If n = 1, then ld1 is defined to be the limit degree ld for ordinary difference fields. Suppose that ldn−1 is defined for difference (σ-) field extensions with Card σ = n − 1. Let L/K be a finitely generated difference field extension with a basic set . . . , αn }, L = KS for a finite set S ⊆ L. For any m ∈ N, let σ = {α1 ,
m Lm = K i=1 αni (S)σ
where σ  = {α1 , . . . , αn−1 }. Then Lm /Lm−1 is a finitely generated σ  -field extension. Applying (ld3) and (ld2) we obtain that ldn−1 (Lm /Lm−1 ) ≥ ldn−1 (Lm+1 /Lm ), hence there exists the limit limm→∞ ldn−1 (Lm /Lm−1 ) = a where a ∈ N or a = ∞. As in the case of ordinary difference fields (consider the proof of Lemma 4.3.2 for n = 1), one can show that a is independent on the choice of σ-generators of L/K. Now we define ldn (L/K) = a. If L/K is not finitely generated, ldn (L/K) is defined to be the maximum of ldn (K  /K) where K  /K is a finitely generated σ-field subextension of L/K, if the maximum exists and ∞ if it does not. Exercise 4.3.26 With the above notation, prove that ldn (M/K) coincides with the limit degree of the extension M/K in the sense of Definition 4.3.

292

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

Exercise 4.3.27 Let K be an ordinary difference field with a basic set σ = {α} and let u, v, w be elements in some σ-overfield of K which are σ-algebraically independent over K. Furthermore, let ξ, η and ζ be solutions of the σ-polynomial A = y 3 +uy 2 +vy+w ∈ Ku, v, w{y} in one σ-indeterminate y in some difference (σ-) field containing K. Prove that if L = Ku, v, w, α(ξ), then A is irreducible in L{y}, ld(Lξ/L) = 1 and ld(Lη/L) = 2.

4.4

The Fundamental Theorem on Finitely Generated Difference Field Extensions

In this section we prove the following result that plays the key role in the study of finitely generated difference field extensions. Theorem 4.4.1 Let K be a difference field with a basic set σ = {α1 , . . . , αn }, M a finitely generated σ-field extension of K, and L an intermediate difference field of M/K. Then the σ-field extension L/K is finitely generated. PROOF. Let {η1 , . . . , ηr } be a difference (σ-) transcendence basis of M/L and let L = Lη1 , . . . , ηr . First, we will show that if the σ-field extension L /K is finitely generated, then L/K is also finitely generated. Indeed, let L = Kλ1 , . . . , λm . Then each λi is a quotient of two polynomials in τ (ηk ) (τ ∈ T, 1 ≤ k ≤ r) with coefficients in L. Let U be the set of all these coefficients. We are going to show that L = KU . If a ∈ L, then a ∈ L , so there exist two polynomials P and Q in τ (ηk ) with coefficients in KU  such that Qa = P . Since the set of all τ (ηk ) (τ ∈ T, 1 ≤ k ≤ r) is algebraically independent over L, the coefficients of the corresponding products of τ ηk in Qa and P must be equal. Since a ∈ L and all the coefficients of P and Q belong to KU , we obtain that a ∈ KU , so L = KU . Let B be a σ-transcendence basis of L /K. Since σ-trdeg(L /K) ≤ σ-trdeg(M/K) < ∞, the set B is finite. Therefore, in order to prove that L/K is finitely generated, it is sufficient to prove that L /KB is finitely generated. Since σ-trdeg(M/KB) = σ-trdeg(M/L) + σ-trdeg(L /KB) = 0 + 0 = 0, without loss of generality we can assume that σ-trdeg(M/K) = 0. The equality σ-trdeg(M/K) = 0 implies that ld(M/K) < ∞ and hence ld(L/K) < ∞. Therefore, there exists a finite set Φ ⊆ L such that ld(L/K) = ld(KΦ/K) and hence ld(L/KΦ) = 1. Clearly, in order to prove the theorem, it is sufficient to show that the σ-field extension L/KΦ is finitely generated, so without loss of generality we can assume that σ-trdeg(M/K) = 0 and ld(L/K) = 1, so that ld(M/K) = ld(M/L). Now, let M = KV  where V is a finite set of σ-generators of M/K. For every j = 1, . . . , n, let σj = σ \ {αj } and for any set S ⊆ M , let KSσj denote the σj -field extension of K (when K is treated as a difference field with the basic set σj .) More general, for any distinct integers i1 , . . . , ik ∈ {1, . . . , n} (1 ≤ k ≤ n), σi1 ,...,ik will denote the set σ \ {αi1 , . . . , αik }, and a σi1 ,...,ik -field extension of K generated by a set S ⊆ M will be denoted by KSσi1 ,...,ik .

4.4. FUNDAMENTAL THEOREM

293

Since ld(M/K) = ld(M/L), there exist p1 , . . . , pn ∈ N such that K(T
(p1 + r1 + 1, p2 + r2 , . . . , pn + rn )V ) : K(T
p1 + r1 , p2 + r2 . . . , pn + rn )(V ) = L(T
(p1 +r1 +1, p2 +r2 , . . . , pn +rn )V ) : L(T
(p1 +r1 , p2 +r2 . . . , pn +rn )V ) = ldk (M/L) < ∞.

(4.4.1)

for all r1 , . . . , rn ∈ N. We are going to show that L ⊆ K

p1 

α1i (V )σ1



···



K

i=0

pn 

αni (V )σn .

(4.4.2)

i=0

Let us denote the set in the right-hand side of (4.4.2) by N and suppose that
∞ L  N ,
so that there exists η ∈ L \ N . Since η ∈ M = K i=0 αni (V )σn and pn αni (V )σn , there exists hn ∈ N such that η∈ / K i=0 pn +h n +1

η ∈ K

pn +hn

αni (V )σn \ K

i=0

αni (V )σn .

(4.4.3)

i=0


∞
pn +hn +1 j
pn +hn +1 i αn (V )σn = K j=0 i=0 αn−1 αni (V )σn,n−1 and Since η ∈ K i=0
pn−1
pn +hn +1 j αn−1 αni (V )σn,n−1 (the last set is contained in N ). η∈ / K j=0 i=0 η ∈ K

∞ 

pn +h n +1

j=pn−1 +hn−1 +1

i=0

∞ 

pn +h n +1

j=pn−1 +hn−1

i=0

K

j αn−1 αni (V )σn,n−1 \

j αn−1 αni (V )σn,n−1 .

(4.4.4)

The inclusions (4.4.3) and (4.4.4) show that η ∈ K(T
(∞, . . . , ∞, pn−1 + hn−1 + 1, pn + hn + 1)V )\ K(T
(∞, . . . , ∞, pn−1 + hn−1 , pn + hn + 1)V ). Proceeding in the same way we find h1 , . . . , hn ∈ N such that η ∈ K(T
(p1 + h1 + 1, p2 + h2 + 1, . . . , pn + hn + 1)V )\ K(T
(p1 + h1 , p2 + h2 + 1, . . . , pn + hn + 1)V ). Since η ∈ L, we have ld(M/K) = K(T
(p1 + h1 + 1, p2 + h2 + 1, . . . , pn + hn + 1)V ) : K(T
(p1 + h1 , p2 +h2 +1, . . . , pn +hn +1)V ) > K(T
(p1 +h1 +1, p2 +h2 +1, . . . , pn +hn +1)V ) :

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

294

K(T
(p1 +h1 , p2 +h2 +1, . . . , pn +hn +1)V ∪{η})) ≥ L(T
(p1 +h1 +1, p2 +h2 +1, . . . , pn + hn + 1)V ) : L(T
(p1 + h1 , p2 + h2 + 1, . . . , pn + hn + 1)V ∪ {η})) = L(T
(p1 +h1 +1, p2 +h2 +1, . . . , pn +hn +1)V ) : L(T
(p1 +h1 , p2 +h2 +1, . . . , pn +hn +1)V ) = ld(M/K) that contradicts (4.4.1). Thus, L ⊆

n  j=1

K

pj 

αji (V )σj .

i=0

Now we can complete the proof by induction on n = Card σ. If n = 0 the statement of the theorem is a classical result of the field theory (see Theorem 1.6.1(ii)). Let n > 0. Since Card σj = n − 1 for j = 1, . . . , n, the induction hypothesis implies that there exist finite sets S1 , . . . , Sn such that pj  # L K αji (V )σj = KSj σj (1 ≤ j ≤ n). Then L = KS1 ∪ · · · ∪ Sn , so i=0

that the σ-field extension L/K is finitely generated.



Because of the importance of Theorem 4.4.1 and in order to illustrate the technique of autoreduced sets of difference polynomials, we present another proof of this theorem based on the results of Section 2.4. ALTERNATIVE PROOF OF THEOREM 4.4.1. As before, let K be a difference field with a basic set σ = {α1 , . . . , αn } and let M = Kη1 , . . . , ηs  be a difference (σ-) field extension of K generated by a finite family {η1 , . . . , ηs }. Furthermore, let L be an intermediate σ-field of the extension M/K. We are going to show that L/K is also a finitely generated σ-field extension. Let P be the defining difference ideal of the s-tuple η = (η1 , . . . , ηs ) in the algebra of σ-polynomials L{y1 , . . . , ys } over L, and let Σ = {A1 , . . . , Ap } be a characteristic set of the σ-ideal P . Let ui , di and Ii denote the leader of Ai , the degree degui Ai , and the initial of the σ-polynomial Ai , respectively (i = 1, . . . , p), and let I(Σ) denote a subset of L{y1 , . . . , ys } consisting of 1 and all finite products of σ-polynomials of the form τ (IA ) where A ∈ Σ, τ ∈ T . Furthermore, let Φ be the set of all coefficients of all σ-polynomials of Σ and let K  = KΦ. Obviously, K  is a finitely generated σ-field extension of K containing in L. We are going to prove the theorem by showing that K  = L. Suppose that λ ∈ L \ K  . Since λ ∈ M = Kη1 , . . . , ηs , there exist two A(η) . Then A(η) − λB(η) = σ-polynomials A, B ∈ K{y1 , . . . , ys } such that λ = B(η) 0 whence the σ-polynomial C = A−λB lies in P . It follows that C reduces to zero modulo [Σ], that is, there exists J ∈ I(Σ) such that JC is a linear combination of σ-polynomials of the form τ (Ai ) (τ ∈ T, 1 ≤ i ≤ p) with coefficients Dτ i of the form Dτ i = Dτ i + λDτi where Dτ i , Dτi ∈ K  {y1 , . . . , ys }. Indeed, let v = τ ui be the Σ-leader of C (τ ∈ T, 1 ≤ i ≤ p), and let v q be the highest power of v that appears in C. Then one can write C as C = (C  + λC  )v q + C 

4.5. ORDINARY DIFFERENCE FIELD EXTENSIONS

295

where the σ-polynomials C  , C  and C  lie in K  {y1 , . . . , ys } and do not contain v e with e > q. The first step of the reduction (described in the proof of Theorem 2.4.1) sends C to the σ-polynomial C1 = (τ Ii )C − v q−di (τ Ai )(C  + λC  ) = [(τ Ii )A − v q−di (τ Ai )C  ] −λ[(τ Ii )B + v q−di (τ Ai )C  ]. Let Ck be a σ-polynomial of the form Ck = Fk −λGk (Fk , Gk ∈ K  {y1 , . . . , ys }) obtained from C after the first k reduction steps, let vk = τ uj be the Σ-leader of Ck (τ ∈ T, 1 ≤ j ≤ p), and let vkqk be the highest power of vk in Ck , so that Ck = (Ck + λCk )vkqk + Ck where the σ-polynomials Ck , Ck and Ck lie in K  {y1 , . . . , ys } and do not contain vke with e > qk . Then the (k + 1)st reduction step transforms Ck into q −dj

Ck+1 = [(τ Ij )F − vkk

q −dj

(τ Aj )Ck ] − λ[(τ Ij )G + vkk

(τ Aj )Ck ].

Since this process reduces C to 0, we obtain that there exist J ∈ I(Σ) such that JC = H1 + λH2 = 0 where H1 and H2 are σ-polynomials in K  {y1 , . . . , ys } that can be written as linear combinations of elements of the form τ (Ai ) (τ ∈ T, 1 ≤ i ≤ p) with coefficients in K  {y1 , . . . , ys }. Moreover, as it is easy to see from the description of the reduction process, H1 is the result of reduction of the σ-polynomial A with respect to Σ, hence H1 = 0 (otherwise A ∈ P and λ = A(η)/B(η) = 0). Now the equality H1 = −λH2 and the fact that H1 , H2 ∈ K  {y1 , . . . , ys } imply that λ ∈ K  . This completes the proof of the theorem.  Corollary 4.5.2 Let K be an inversive difference field with a basic set σ, M a finitely generated σ ∗ -field extension of K, and L and intermediate σ ∗ -field of M/K. Then the σ ∗ -field extension L/K is finitely generated, that is, there exists a finite set V ⊆ L such that L = KV ∗ . PROOF. Let S be a finite set of σ ∗ -generators of M/K, so that M = KS∗ . Then the chain K ⊆ L ∩ KS ⊆ KS can be considerd as a sequence of σ-field extensions. By Theorem 4.4.1, L ∩ KS is a finitely generated σ-field extension of K, so that there exists a finite set V ⊆ L such that L ∩ KS = KV . Let us show that L = KV ∗ . Indeed, if a ∈ L, then a ∈ M = KS∗ , hence there exists τ ∈ T such that τ (a) ∈ KS. Therefore, τ (a) ∈ L ∩ KS = KV  hence a ∈ KV ∗ . Since the inclusion KV ∗ ⊆ L is obvious, we obtained the desired equality. 

4.5

Some Results on Ordinary Difference Field Extensions

Let K be an ordinary difference field with a basic set σ = {α} and let L be a difference overfield of K. In what follows, in accordance with the terminology and notation introduced by R. Cohn, the transcendence degrees trdegK L and

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trdegK ∗ L∗ will be also called the order and effective order of the σ-field extension L/K; these characteristics will be denoted by ord G/F and Eord G/F , respectively. (As usual, K ∗ denotes the inversive closure of a difference field K.) It is easy to check that Eord L/K ≤ ord L/K, Eord L/K = ord L/K if K is inversive, and ord L/K = ∞ if σ-trdegK L > 0 (we leave the verification to the reader as an exercise). Furthermore, the properties of transcendence degree imply that for any chain K ⊆ L ⊆ M of ordinary difference field extensions we have ord M/K = ord M/L + ord L/K and Eord M/K = Eord M/L + Eord L/K. Recall that an ordinary difference field K with a basic set σ = {α} is called aperiodic if there is no integer k ≥ 1 such that αk (a) = a for all a ∈ K. We say that K is completely aperiodic if either Char K = 0 and the σ-field K is aperiodic, or if Char K = p > 0 and the field K satisfies the following condition: if q and r are powers of p, and i, j ∈ N, then (αi (a))q = (αj (a))r for all a ∈ K if and only if i = j and q = r. Exercise 4.5.1 With the above notation, show that if an aperiodic σ-field K of positive characteristic has infinitely many invariant elements, then K is completely aperiodic. [Hint: Show that if Char K = p > 0 and r, s ∈ N, r = s, then for any i, j ∈ N r there are only finitely many invariant elements a ∈ K such that (αi (a))p = s (αj (a))p .] The complete aperiodicity plays an important role in R. Cohn’s theorems on simple difference field extensions which are the central results of this section. Note that the direct generalization of the theorem on primitive element of a finitely generated separable algebraic field extension (Theorem 1.6.17) to the difference case is not true. The following counterexample is presented in [34]. Example 4.5.2 Let c1 , . . . , cm (m > 1) be elements in some field extension of Q which are algebraically independent over Q. Let K denote the field Q(c1 , . . . , cm ) treated as an ordinary difference field whose basic set σ consists of the identity automorphism α. Furthermore, let P denote the reflexive difference ideal generated by the σ-polynomials αy − ci (1 ≤ i ≤ m) in the ring K{y} (as usual, K{y} denotes the ring of σ-polynomials in one σ-indeterminate y over K). It follows from Proposition 2.4.9 that P is a reflexive prime σ-ideal, so it has some generic zero (η1 , . . . , ηm ) with coordinates in some σ-overfield of K. Let L = Kη1 , . . . , ηm . Then α(ηi ) = ci ∈ K for i = 1, . . . , m, hence every element of L is a solution of some σ-polynomial of first order in K{y}. Applying Corollary 4.1.18 we obtain that if the σ-field extension L/K is generated by a single element, then trdegK L = 1. However, it is easy to see that trdegK L = m > 1. (If there is a polynomial f in m variables over K such that f (η1 , . . . , ηm ) = 0, then one can apply α to both sides of the last equality and obtain that the elements c1 , . . . , cm are algebraically dependent over Q, contrary to the choice of these elements.) Thus, L is a finitely generated σ-field extension of a difference (σ-) field K of zero characteristic, every element of L is σ-algebraic over K, but there is no element ζ ∈ L such that L = Kζ.

4.5. ORDINARY DIFFERENCE FIELD EXTENSIONS

297

Proposition 4.5.3 Let K be a completely aperiodic difference subfield of an ordinary difference field L with a basic set σ = {α}. Let K{y1 , . . . , ys } be the algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys over K and let A be a nonzero σ-polynomial in K{y1 , . . . , ys }. Then there exists and s-tuple η = (η1 , . . . , ηs ) with coordinates in L such that A(η) = 0. PROOF. First of all, elementary induction arguments show that it is sufficient to consider the case s = 1. Thus, we assume that A is a nonzero σ-polynomial in K{y} (y is a σ-indeterminate over K). If A is linear, but not homogeneous, then A(0) = 0. Suppose that A is a linear homogeneous σ-polynomial, so that A has a representation of the form A = a1 αi1 y + · · · + ar αir y

(4.5.5)

where ai are nonzero elements of K and i1 , . . . , ir ∈ N. We are going to use induction on r to show that there is η ∈ L such that A(η) = 0. If r = 1, then A(1) = 0. Suppose that r > 1 and the statement is true for linear homogeneous σ-polynomials with fewer than r terms in the representation of the form (4.5.5). Let u be an element of L such that αir (u) = αir−1 (u), and let A is the σ-polynomial in K{y} obtained by replacing y with uy everywhere in A. Then the nonzero σ-polynomial B = αir (u)A − A has fewer terms than A. Therefore, there exists ζ ∈ L such that B(ζ) = 0, hence ζ cannot annul both A and A . It follows that either A(ζ) = 0 or A(uζ) = 0. Now let A be arbitrary σ-polynomial in K{y} of degree d > 1. We proceed by induction of d. The proposition has been proved for d = 1, and we assume that it is true for σ-polynomials of degree less than d. Let K{y, z} be the algebra of σ-polynomails in two σ-indeterminates y and z over K. In what follows, for any D ∈ K{y, z}, the total degrees of D relative to the sets of variables {αk y | k ∈ N} and {αk z | k ∈ N} will be denoted by deg{y} D and deg{z} D, respectively. Our induction hypothesis implies that if deg{y} D < deg A(= deg{y} A) and deg{z} D < deg A, then there exist elements η, ζ ∈ L such that D(η, ζ) = 0. Let F and G be two σ-polynomials in K{y, z} obtained from A by replacing y with y + z and z, respectively. Then F = A + G + C where either C = 0 or C = 0 and deg{y} C < deg A, deg{z} C < deg A. Note that if Char K = 0, then C = 0. Indeed, in this case not every formal partial derivative ∂A/∂αk y is 0, so the formal Taylor expansion of F in powers of variables αk z (k ∈ N) contains a nonzero summand of the form (∂A/∂αi y)αi z. This summand contains all terms of A which are of positive degree with respect to the set of transforms of y and linear with respect to αi z. Therefore, such a term cannot be cancelled by any terms from the other summands in the Taylor expansion. Also, such a term cannot be in A or G, so it must be in C whence C = 0. Now we can complete the proof for the case C = 0. Let η and ζ be two elements in L such that C(η, ζ) = 0 (such elements exists by the induction hypothesis). If A(η) = 0 and A(ζ) = 0, then F (η, ζ) = C(η, ζ) = 0. Therefore, one of the elements η, ζ, or η + ζ does not annul A.

298

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It remains to consider the case Char K = p > 0, C = 0. Let M be a monomial of A, that is, a power product of the form (αk1 y)l1 . . . (αkq y)lq which appears in A with a nonzero coefficient. If αi y and αj y, i = j, appear in M , then M contributes to F a term with monomial M  obtained from M by the substitution of αi z for αi y. This term cannot be cancelled by any other terms and is in C. Thus, every monomial M of A is of the form M = (αi y)d . Suppose r that d = kpr where k is not divisible by p. Then a term ((k(αi y)k−1 )p appears i i d in (α y + α z) and contributes a term to F . If k = 1, this term is in C. Since s  t  j C = 0, we have the only possibility: A = aij (αi y)p where the coefficients i=1 j=1

aij lie in K. Now one can apply the argument used in the first part of the proof (when we considered the case of a linear σ-polynomial A) and obtain (exploring the complete aperiodicity instead of the aperiodicity) that there is an element η ∈ L such that A(η) = 0.  Theorem 4.5.4 Let K be a completely aperiodic ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K such that σ-trdegK L = 0. If Char K = 0 or Char K = p > 0 and rld(L/K) = ld(L/K), then there exists an element θ ∈ L and a non-negative integer k such that αk (a) ∈ Kθ for any a ∈ L. Moreover, the element θ may be chosen as a linear combination of the members of any finite set of σ-generators of L/K with coefficients from any pre-assigned completely aperiodic σ-subfield of K. PROOF. We start with the case Char K = 0 and L = Kη, ζ (obviously, it is sufficient to prove the result for the case of two σ-generators of L/K). Let u be an element in some σ-overfield of L which is σ-transcendental over L. Let v = η + ζu. Then v is σ-algebraic over Ku, hence there exists s, k ∈ N such that αk (v) is algebraic over K(u, . . . , αs (u); v, . . . , αk−1 (v)). Let us choose the smallest possible k with this property. Then k = trdegK u Ku, v ≤ trdegK u Ku, η, ζ ≤ trdegK u Kη, ζ. Let A be a nonzero polynomial in s + k + 2 variables X1 , . . . , Xs+k+2 with coefficients in K such that A(u, . . . , αs (u); v, . . . , αk (v)) = 0 and A is of the lowest possible degree in Xs+k+2 (the variable which is replaced with αk (v) in the last equality). Since the elements u, . . . , αs (u) are algebraically independent over Kη, ζ, the coefficients of the power products of the αi (u) in the expressions obtained by replacing each αj (v), 0 ≤ j ≤ k, in A with αj (η) + αj (ζ)αj (u) and expanding formally are all equal to 0. Therefore, the formal partial derivative of this expansion with respect to αk (u) is 0, that is, ∂A/∂(αk (u)) + αk (ζ)∂A/∂(αk (v)) = 0. Because of the minimum conditions on k and degree of A with respect to Xs+k+2 , ∂A/∂(αk (v)) = 0. It follows that αk (ζ) = −(∂A/∂(αk (u)))/(∂A/∂(αk (v))), hence αk (ζ) and, therefore, also αk (η) belong to Ku, v. Thus, one can write

4.5. ORDINARY DIFFERENCE FIELD EXTENSIONS

299

αk (η) = F/H and αk (ζ) = G/H where F , G, and H are σ-polynomials in u and v with coefficients in K and H = 0. (More precisely, there are σ-polynomials ˜ and H ˜ in the algebra of σ-polynomials K{y, z} in two σ-indeterminates F˜ , G, y and z over K such that the above expressions for αk (η) and αk (ζ) hold with ˜ v), and H = H(u, ˜ F = F˜ (u, v), G = G(u, v).) Let H be written as a σ-polynomial in u with coefficients in Kη, ζ. By Proposition 4.5.3, there exists an element µ ∈ K such that H is not annulled when u is replaced by µ. Let θ = η = µζ. We are going to show that θ and k have the properties stated in the theorem. Let F, G, H become F  , G , H  , respectively, when u is replaced by µ and v by θ. Then H  = 0. Clearly, it is sufficient to show that H  αk (η) = F  and H  αk (ζ) = G . (These equalities would imply that αk (η), αk (ζ) ∈ Kθ and, therefore, the inclusion αk (x) ∈ Kθ for any x ∈ L.) Considering the equality Hαk (η) = F one can see that both its sides can be viewed as σ-polynomials in u with coefficients in Kη, ζ. Since u is σ-transcendental over Kη, ζ, corresponding coefficients on both sides are equal. therefore, the equality is preserved if u is replaced by µ. Rewriting both sides of the resulting equality as appropriate combinations of µ and η + µζ, we obtain that H  αk (η) = F  . Similarly H  αk (ζ) = G . This completes the proof in the case of characteristic 0. Suppose that Char K = p > 0 and ld(L/K) = rld(L/K). Since the reduced limit degree of a difference field extension cannot exceed its limit degree, one can apply Theorem 4.3.4 and obtain that for every intermediate σ-field L of the extension L/K we have ld(L /K) = rld(L /K). Therefore, we may assume, as before, that L = Kη, ζ for some elements η, ζ ∈ L. Let u be a σ-transcendental over L element in some σ-overfield of L, and let θ = η + uζ. If Φ and Ψ are subsets of L such that Φ ⊆ Ψ then the statements of Theorem 1.6.23(viii) and Theorem 1.6.28(vi) imply that Ku(Ψ) : Ku(Φ) = K(Ψ) : K(Φ) and [Ku(Ψ) : Ku(Φ)]s = [K(Ψ) : K(Φ)]s . Therefore, ld(Lu/Ku) = ld(L/K) and rld(Lu/Ku) = rld(L/K), hence ld(Lu/Ku) = rld(Lu/Ku). Furthermore, by the argument used at the beginning of this paragraph, we obtain that ld(Ku, v/Ku) = rld(Ku, v/Ku). It follows that there exists s, k ∈ N such that αk (v) is algebraic over the field K(u, . . . , αs (u); v, . . . , αk−1 (v) ), and the degree and separable factor of the degree of αk (v) over this field are equal. Then αk (v) is separably algebraic over K(u, . . . , αs (u); v, . . . , αk−1 (v)). Let Z be an indeterminate over the field K(u, . . . , αs (u); v, . . . , αk−1 (v) ) and let f = f (Z) be an irreducible polynomial in the polynomial ring K(u, . . . , αs (u); v, . . . , αk−1 (v) )[Z] such that f (αk (v)) = 0 and the coefficients of f belong to K[u, . . . , αs (u); v, . . . , αk−1 (v)] (the existence of such a polynomial follows from the conclusion made at the end of the previous paragraph). Since αk (v) is separably algebraic over K(u, . . . , αs (u); v, . . . , αk−1 (v)), the formal partial derivative ∂C/∂Z is not 0, so that ∂C/∂Z is not annulled by αk (v). Let D be the polynomial expression in u, . . . , αs (u); v, . . . , αk (v) obtained by replacing Z in f with αk (v). Then we can complete the proof of the first statement of the theorem by repeating the argument of our proof in the case

300

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Char K = 0 where A is replaced by D and the fact that (∂C/∂Z)(αk (v)) = 0 is used to show that (∂D/∂(αk (v))) = 0. It follows from Proposition 4.5.3 that the element µ in the first part of our proof may be chosen from any completely aperiodic σ-subfield of K. This remark proves the last statement of the theorem. 

4.6

Difference Algebras

Let K be a difference (respectively, inversive difference) ring with a basic set σ. A K-algebra R is said to be a difference algebra over K or a σ-K-algebra (respectively, an inversive difference algebra over K or a σ ∗ -K-algebra) if elements of σ (respectively, σ ∗ ) act on R in such a way that R is a σ- (respectively, σ ∗ -) ring and α(au) = α(a)α(u) for any a ∈ K, u ∈ R, α ∈ σ (for any α ∈ σ ∗ if R is a σ ∗ -K-algebra). A σ-K- (respectively, σ ∗ -K-) algebra R is said to be finitely generated if there exists a finite family {η1 , . . . , ηs } of elements of R such that R = K{η1 , . . . , ηs } (respectively, R = K{η1 , . . . , ηs }∗ ). If a σ-K- (or σ ∗ -K-) algebra R is an integral domain, then the σ-transcendence degree of R over K is defined as the σ-transcendence degree of the corresponding σ- (respectively, σ ∗ -) field of quotients of R over K. In what follows we consider some applications of the theorems on finitely generated difference field extensions to difference algebras. We will formulate and prove some results about inversive difference algebras over inversive difference fields, leaving to the reader the formulations and proofs of the corresponding statements for difference and “general difference” cases as exercises. (As before, by a “general difference” case we mean the study of a difference ring (field, module, algebra) R whose basic set is a union of a finite set σ of injective endomorphisms of R and a finite set  of automorphisms of R such that any two elements of σ ∪  commute, see Section 3.6. for details.) Let R be an inversive difference algebra over an inversive difference field K with a basic set σ = {α1 , . . . , αn } and let U denote the set of all prime σ ∗ ideals of R. As in Section 3.6 (see Proposition 3.6.16), one can consider the set BU = {(P, Q) ∈ U × U|P ⊇ Q} and the uniquely defined mapping µU : BU → Z such that (i) µU (P, Q) ≥ −1 for every pair (P, Q) ∈ BU ; (ii) for any d ∈ N, the inequality µU (P, Q) ≥ d holds if and only if P = Q and there exists an infinite chain P = P 0 ⊇ P1 ⊇ · · · ⊇ Q such that Pi ∈ U and µU (Pi−1 , Pi ) ≥ d − 1 for i = 1, 2, . . . . Definition 4.6.1 With the above notation, the least upper bound of the set {µU |(P, Q) ∈ BU } is called the type of the σ ∗ -K-algebra R. The least upper bound of the lengths k of chains P0 ⊇ P1 ⊇ · · · ⊇ Pk , such that P0 , . . . , Pk ∈ U and µU (Pi−1 , Pi ) = typeU R (i = 1, . . . , k), is called the dimension of R.

4.6. DIFFERENCE ALGEBRAS

301

The type and dimension of a σ ∗ -K-algebra R are denoted by typeU R and dimU R, respectively. Theorem 4.6.2 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, R = K{η1 , . . . , ηs }∗ a σ ∗ -K-algebra without zero divisors generated by a finite set η = {η1 , . . . , ηs }, and U the family of all prime σ ∗ -ideals of R. Then: (i) typeU R ≤ n. (ii) If σ-trdegK R = 0, then typeU R < n. (iii) If typeU R = n, then dimU R ≤ σ-trdegK R. (iv) If η1 , . . . , ηs are σ-algebraically independent over K, then typeU R = n and dimU R = s. PROOF. Let Γ be the free commutative group generated by the set σ and for any r ∈ N, let Γ(r) = {γ ∈ Γ | ord γ ≤ r} (recall n that the order of an element γ = α1k1 . . . αnkn ∈ Γ is defined as ord γ = i=1 |ki |). Furthermore, let Rr (r ∈ N) denote the K-algebra K[{γ(ηj ) | γ ∈ Γ(r), 1 ≤ j ≤ s}] and let Rr = R for r ∈ Z, r < 0. Let P be a prime σ ∗ -ideal of R and let η i denote the canonical image of ηi in the factor ring R/P (1 ≤ i ≤ s). It is easy to see that for every r ∈ N, P ∩ Rr is a prime ideal of the ring Rr and the quotient fields of the rings Rr /P ∩ Rr and K[{γ(η j ) | γ ∈ Γ(r), 1 ≤ j ≤ s}] are isomorphic. By Theorem 4.2.5, there exists a numerical polynomial ψ(t) in one variable t such that ψ(t) = trdegK K[{γ(ηj ) | γ ∈ Γ(r), 1 ≤ j ≤ s}] = trdegK (Rr /P ∩ Rr ) for all sufficiently large r ∈ Z, deg ψ(t) ≤ n and the polynomial ψ(t) can be written as 2n aP n t + o(tn ) ψ(t) = n! where aP = σ-trdegK (R/P ). It is easy to see that if P, Q ∈ U (as above, U denote the set of all prime σ ∗ -ideals of R) and P ⊇ Q, then ψP (t)  ψQ (t) where  denotes the order on the set of all numerical polynomials introduced in Definition 1.5.10 (by Theorem 1.5.11, the set W of all Kolchin polynomials is well-ordered with respect to ). Furthermore, if P, Q ∈ U and P  Q, then ψP (t) ≺ ψQ (t). Indeed, in this case there exists r0 ∈ N such that P ∩Rr  Q∩Rr for all r ≥ r0 . Since trdegK (Rr /P ∩Rr ) and trdegK (Rr /Q∩Rr ) are, respectively, the coheights of the ideals P ∩ Rr and Q ∩ Rr in the ring Rr , Theorem 1.2.25(ii) shows that for all sufficiently large r ∈ Z, ψP (r) = trdegK (Rr /P ∩ Rr ) = coht(P ∩ Rr ) < coht(Q ∩ Rr ) = trdegK (Rr /Q∩Rr ) = ψQ (r). It follows that if P, Q ∈ U, P ⊇ Q and ψP (t) = ψQ (t), then P = Q. Now the arguments of the proof of Theorem 3.6.24 show that for any d ∈ Z, d ≥ −1

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302

and for any pair (P, Q) ∈ BU (that is, for any P, Q ∈ U such that P ⊇ Q), the inequality µU (P, Q) ≥ d implies the inequality deg(ψQ (t) − ψP (t)) ≥ d. Since ψP (t) ≤ n for any prime σ ∗ -ideal P in R and ψP (t) < n if σ-trdegK R = 0 (in this case aP = 0), we have deg(ψQ (t)−ψP (t)) ≤ n for any pair (P, Q) ∈ BU and this inequality is strict if σ-trdegK R = 0. Therefore, for every (P, Q) ∈ BU , we have µU (P, Q) ≤ n, and if σ-trdegK R = 0 then the last inequality is always strict. It follows that typeU R ≤ n and typeU R < n if σ-trdegK R = 0. In order to prove statement (iii), suppose that typeU R = n and P0  P1  · · ·  Pd (d ∈ N) is a descending chain of prime σ ∗ -ideals of R such that µU (Pi−1 , Pi ) = typeU R = n for i = 1, . . . , d. Clearly, in order to prove the inequality dimU R ≤ σ-trdegK R, it is sufficient to show that d ≤ σ-trdegK R. Since µU (Pi−1 , Pi ) = n (1 ≤ i ≤ d), the above considerations show that deg(ψPi (t) − ψPi−1 (t)) ≥ n hence aPi − aPi−1 ≥ 1 for i = 1, . . . , n. Now the equalities  d d  n   2 (aPi − aPi−1 )tn + o(tn ) [ψPi (t) − ψPi−1 (t)] = ψPd (t) − ψP0 (t) = n! i=1 i=1 =

2n (aPd − aP0 )tn + o(tn ) n!

imply that aPd − aP0 =

d 

(aPi − aPi−1 ) ≥ d.

i=1

On the other hand, ψPd (t) − ψP0 (t) ≤ ψ(0) (t) − ψP0 (t) ≤ ψ(0) (t) = ψη|K (t)

(4.6.1)

where ψη|K (t) is the σ ∗ -dimension polynomial of the extension Kη1 , . . . , ηs ∗ /K associated with the system of σ ∗ -generators η. Since ψη|K (t) =

2n ∗ (σ -trdegK R)tn + o(tn ) n!

(see Theorem 4.2.5), inequalities (4.6.1) show that d ≤ aPd −aP0 ≤ σ ∗ -trdegK R. Thus, dimU R ≤ σ-trdegK R if typeU R = n. Suppose that η1 , . . . , ηs are σ-algebraically independent over K. Let L denote the set of all linear σ ∗ -ideals of R. (Recall that by a linear σ ∗ -ideal of R we mean an ideal I generated (as a σ ∗ -ideal) by homogeneous linear σ ∗ -polynomials of m  ai γi yki with ai ∈ K, γi ∈ Γσ (1 ≤ ki ≤ s for i = 1, . . . , m). ) Since the form i=1

L ⊆ U (see Proposition 2.4.9), typeL R ≤ typeU R ≤ n. Therefore, in order to prove the equality typeU R = n, it is sufficient to show that typeL R ≥ n. This inequality, in turn, is a consequence of the inequality µL ([η1 , . . . , ηp ]∗ , [η1 , . . . , ηp−1 ]∗ ) ≥ n we are going to prove.

(p = 1, . . . , s)

(4.6.2)

4.6. DIFFERENCE ALGEBRAS

303

Let L(p) denote the set of all linear σ ∗ -ideals P ∈ L such that [η1 , . . . , ηp−1 ]∗ ⊆ P ⊆ [η1 , . . . , ηp ]∗ . Then inequality (4.6.2) is equivalent to the inequality µL(p) ([η1 , . . . , ηp ], [η1 , . . . , ηp−1 ]) ≥ n

(p = 1, . . . , s).

(4.6.3)

Furthermore, the canonical ring σ ∗ -isomorphism K{η1 , . . . , ηs }∗ /[η1 , . . . , ηp−1 ]∗ ∼ = K{ηp , . . . , ηs }∗

(p = 1, . . . , s) (p)

induces a bijection of the family L(p) onto the family L1

of all linear σ ∗ q  ideals of K{ηp , . . . , ηs }∗ whose σ ∗ -generators are of the form aj γj (ηp ) with j=1

aj ∈ K, γj ∈ Γ (q ≥ 1). Of course, this bijection preserves inclusions of σ ∗ ideals. (p) Let L2 (1 ≤ p ≤ s) be the family of all linear σ ∗ -ideals of the σ ∗ -K(p) (p) algebra K{ηp }∗ . It is easy to see that if J ∈ L2 , then JK{ηp , . . . , ηs }∗ ∈ L1 and JK{ηp , . . . , ηs }∗ ∩ K{ηp }∗ = J, so there is a one-to-one correspondence (p) (p) between the families L1 and L2 that preserves inclusions. Thus, in order to prove inequality (4.6.3) (and, therefore, inequality (4.6.2)) it is sufficient to show that if K{y}∗ is an algebra of σ ∗ -polynomials in one σ ∗ -indeterminate y over K and B is the family of all linear σ ∗ -ideals of K{y}∗ , then typeB K{y}∗ ≥ n. We will prove this inequality by induction on n. If n = 0, the inequality is obvious. Let n > 0 and let σn = σ \ {αn } = {α1 , . . . , αn−1 }. Let us consider the descending chain [y]∗  [(αn − 1)y]∗  · · ·  [(αn − 1)r y]∗ · · ·  (0) of σ ∗ -ideals of K{y}∗ and let Br (r ∈ N) be the set of all σ ∗ -ideals I ∈ B such that [(αn − 1)r+1 y]∗ ⊆ I ⊆ [(αn − 1)r y]∗ . Denoting the canonical image of the element (αn − 1)r y in the σ ∗ -ring K{(αn − 1)r y}∗ /[(αn − 1)r+1 y]∗ by zr (r = 1, 2, . . . ), we can treat the last ring as a ring of σn∗ -polynomials K{zr }∗σn in one σn∗ -indeterminate zr over K (the index σn indicates that this ring, as well as the field K, is considered with respect to the basic set σn ). Notice that K{zr }∗σn is also a σ ∗ -ring where αn (zr ) = zr . Let Γ be the subgroup of Γ generated by the set σn∗ and let Ar denote the family of all σn∗ -ideals of K{zr }∗σn generated by sets of elements of the form γ  zr with γ  ∈ Γ . Then µBr ([(αn − 1)r y]∗ , [(αn − 1)r+1 y]∗ ) = µAr ([zr ]∗σn , (0)), so the inductive hypothesis leads to the inequality µBr ([(αn − 1)r y]∗ , [(αn − 1)r+1 y]∗ ) ≥ n − 1.

304

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Therefore, µBr ([y]∗ , (0) ) ≥ n − 1 for every r ∈ N hence typeB K{y}∗ ≥ n. We have proved inequality (4.6.2) and therefore the equality typeU R = n. Moreover, the above arguments show that if η1 , . . . , ηs are σ-algebraically independent over K, then µU ([η1 , . . . , ηp ]∗ , [η1 , . . . , ηp−1 ]∗ = typeU R = n for p = 1, . . . , s, hence dimU R ≥ s. Combining this inequality with statement  (iii) of our theorem we obtain that dimU R = s. Exercise 4.6.3 Formulate and prove an analog of Theorems 4.6.2 for difference field extensions. We conclude this section with some results on local difference algebras. In what follows, all fields are supposed to have zero characteristics. Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let a σ ∗ -K-algebra A be a local ring with a maximal ideal m. Clearly, m is a σ ∗ -ideal of A, the residue field k = A/m can be naturally considered as a σ ∗ -field extension of K and m/m2 can be treated as a vector σ ∗ -k-space. Proposition 4.6.4 Let K be an inversive difference field with a basic set σ and let A be a local σ ∗ -K-algebra with a maximal ideal m. Furthermore, let k denote the residue field A/m (treated as a σ ∗ -overfield of K). Then there exists an exact sequence of vector σ ∗ -k-spaces ν ρ → ΩA|K k− → Ωk|K → 0 (4.6.4) 0 → m/m2 − A

where the σ-homomorphisms ρ and ν act as follows: ρ(x + m2 ) = dA|K x 1, ν(dA|K y 1) = dk|K y¯

(4.6.5)

for every x ∈ m, y ∈ A (¯ y is the coset of y in the factor ring A/m). PROOF. Since K is a field, every exact short sequence of K-modules splits. By Theorem 1.7.19(iii), there exists an exact sequence of vector σ ∗ -k-spaces ρ

→ ΩA|K 0 → m/m2 −



ν

k− → Ωk|K → 0

A

where the actions of σ-homomorphisms ρ and ν are defined by conditions (4.6.5). Let Ek and EA denote the rings of inversive difference (σ ∗ -) operators over k and A, respectively. Then m/m2 can be naturally considered as a left Ek -module, while Lemma 4.2.8 shows that Ωk|K and ΩA|K can be naturally - modules, respectively. As we have seen in Section 3.4, treated as left Ek - and EA the vector k-space ΩA|K A k can be viewed as a left Ek -module where elements of the free commutative group  Γ generated by the set σ act in such a way that γ(dA|K x a) = γ(dA|K x) γ(a) for every γ ∈ Γ, x ∈ A, a ∈ k. It remains

4.6. DIFFERENCE ALGEBRAS

305

to show that the homomorphisms of vector k-spaces ρ and ν are actually σ-homomorphisms, that is, 1)) = γ(ν(dA|K η 1)) (4.6.6) ρ(γ x ¯) = γρ(¯ x), ν(γ(dA|K η for any γ ∈ Γ, x ¯ = x + m2 ∈ m/m2 , η ∈ A. Indeed, if γ ∈ Γ, then we have ρ(γ x ¯) = ρ(γ(x) + m2 ) = dA|K γ(x) 1 = γdA|K x 1 = γρ(¯ x) and ν(γ(dA|K η



1)) = ν(dA|K γ(η)



1) = dK|k γ(¯ η ) = γ(ν(dA|K η



1))

where η¯ denotes the canonical image of η in the field k = A/m. Thus, the mappings ρ and ν in (4.6.4) are σ-homomorphism of vector σ ∗ -k-spaces. This completes the proof.  In what follows, if A is a local algebra with a maximal ideal m over a field K and B a K-subalgebra of A, then Bm will denote the set of all elements of A f f / m. (We write for which can be written in the form where f, g ∈ B and g ∈ g g f g −1 taking into account that elements of A \ m are units of A.) Clearly, Bm is a local K-subalgebra of A with the maximal ideal m ∩ Bm . Definition 4.6.5 Let K be an inversive difference field with a basic set σ and let A be a local σ ∗ -K-algebra without zero divisors. We say that A is a local σ ∗ K-algebra of finitely generated type if there exist elements η1 , . . . , ηs ∈ A such that A = K{η1 , . . . , ηs }∗m where m is the maximal ideal of A. Theorem 4.6.6 Let K be an inversive difference field with a basic set σ and let an integral domain A be a local σ ∗ -K-algebra of finitely generated type with a maximal ideal m and the residue field k = A/m. Then (i) m/m2 is a finitely generated vector σ ∗ -K-space. (ii) σ ∗ -dimk m/m2 ≥ σ ∗ -trdegK k. PROOF. Let EA and Ek denote the rings of σ ∗ -operators over A and k, respectively, and let ν ρ → ΩA|K k− → Ωk|K → 0 0 → m/m2 − A

be the exact sequence of E k -modules considered in Proposition 4.6.4. We are going to show that ΩA|K A k is a finitely generated Ek -module. (Since the ring Ek is left Noetherian (see Theorem 3.4.2), we shall obtain that m/m2 is also a finitely generated Ek -module.) By the assumption of the theorem, there exist elements η1 , . . . , ηs ∈ A such that A = K{η1 , . . . , ηs }∗m . Let us show that every element dA|K ξ (ξ ∈ A) can

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306

be written as a linear combination of elements dA|K η1 , . . . , dA|K ηs with cof (η1 , . . . , ηs ) efficients in EA . Indeed, any element ξ ∈ A can be written as g(η1 , . . . , ηs ) where f and g are σ ∗ -polynomials in σ ∗ -indeterminates y1 , . . . , ys over K and / m. Since dA|K f (η1 , . . . , ηs ) and dA|K g(η1 , . . . , ηs ) can be writg(η1 , . . . , ηs ) ∈ ten as linear combinations of elements dA|K γ(ηi ) = γdA|K ηi (γ ∈ Γ, 1 ≤ 1 [g(η1 , . . . , ηs )dA|K f (η1 , . . . , ηs ) − i ≤ s), the element dA|K ξ = 2 g (η1 , . . . , ηs ) f (η1 , . . . , ηs )dA|K g(η1 , . . . , ηs )] can be written as such a linear combination as well.    ηs 1 generate ΩA|K A k as It follows that elements dA|K η1 1,. . . , dA|K a left  Ek -module. Indeed, any generator dA|K η A (η ∈ A) of the Ek -module ΩA|K A k can be written in the form ri s   i=1 j=1

aij γij dA|K ηi



1=

ri s   ( a ¯ij γij )(dA|K ηi 1) i=1 j=1

¯ij denotes the image of aij under where aij ∈ A (1 ≤ i ≤ s, 1 ≤ j ≤ ri ) and a ri  a ¯ij γij ∈ Ek (1 ≤ i ≤ s), the the natural epimorphism A → k = A/m. Since j=1    elements dA|K η1 1, . . . , dA|K ηs 1 generate the Ek -module ΩA|K A k. This completes the proof of statement (i). Now let us prove the second statement of the theorem. As before, for any r ∈ N let Γ(r) denote the set {γ ∈ Γ | ord γ ≤ r}, and for any η ∈ A let Γ(r)η = {γ(η) | γ ∈ Γ(r)}. Furthermore, let Ar denote the local subring K[Γ(r)η1 ∪ · · · ∪ Γ(r)ηs ]m of Ar , let mr = m ∩ Ar , and let kr denote the residue field Ar /mr of the ring Ar . First of all, one can apply Theorem 1.2.25 (ii), (v) and obtain the following inequality.

dimkr (mr /m2r ) ≥ trdegK Ar − trdegK kr

(4.6.7)

for every r ∈ N. Now, let K{y1 , . . . , ys }∗ denote the ring of σ ∗ -polynomials in σ ∗ -indeterminates y1 , . . . , ys over K, let π : K{y1 , . . . , ys }∗ → K{η1 , . . . , ηs }∗ be the natural σ-epimorphism of σ ∗ -K-algebras (π : yi → ηi for i = 1, . . . , s and π(a) = a for every a ∈ K), and let P = Ker π. Furthermore, let B denote the local σ ∗ -K-algebra K{y1 , . . . , ys }∗P and let n denote the maximal ideal of B. It is easy to see that the homomorphism π can be naturally extended to a σ-homomorphism of local σ ∗ -K-algebras π  : B → A such that π  (n) = m. Let Q = P B and let Br (r ∈ N) denote the local subring K[Γ(r)y1 ∪· · ·∪Γ(r)ys ]n of B. Also, for any r ∈ N and M ⊆ B, let Mr denote the set M ∩Br (in particular, we set nr = n ∩ Br ). In what follows we identify the field Br /nr with the field kr via the σ-isomorphism naturally induced by π  ).

4.6. DIFFERENCE ALGEBRAS

307

By Theorem 1.7.19, for every r ∈ N, there exists an exact sequence kr → Ωkr |K (4.6.8) 0 → nr /n2r → ΩBr |K Br

and a commutative diagram with exact rows   - ΩBr |K B k 0 - nr /n2r kr k r jr ? 

ir ? -

 kr

k - 0

hr ?

(4.6.9)

- 0 Ωk|K  where the first row is obtained by applying the functor kr k to the exact sequence (4.6.8). We are going to show that the mapping jr in diagram (4.6.9) is injective. To obtain this result we first notice that it is sufficient to show that for every vector k-space V , the induced k-homomorphism of vector k-spaces k, V ) → Homk (ΩBr |K k, V ) jr∗ : Homk (ΩB|K 0

-

- Ωkr |K

n/n2

ΩB|K

B

k

-

B

Br

is surjective. This, however, is equivalent to asserting that the canonical map DerK (B, V ) → DerK (Br , V ) is surjective, see Corollary 1.7.12 which implies the existence of sequences of natural isomorphisms k, V ) ∼ Homk (ΩB|K = HomB (ΩB|K , V ) ∼ = DerK (B, V ) B

and Homk (ΩBr |K



k, V ) ∼ = HomB (ΩBr |K , V ) ∼ = DerK (Br , V ).

Br

Since B is the ring of quotients of a polynomial extension of the ring Br , every derivation from B to a vector k-space V can be extended to a derivation from B to V (see Proposition 1.7.9). It follows that the map jr∗ is surjective hence jr is injective. Furthermore, the injectivity of jr in diagram (4.6.9) implies that the map ir in diagram (4.6.9)is also injective. Since elements dB|K (γyi ) 1 (γ ∈ Γ, 1 ≤ i ≤ s) generate the vector k-space Im ir (r ∈ N), (Im ir )r∈N is an excellent filtration of the vector σ ∗ -k-space n/n2 (we consider a filtration with r ∈ N as a filtration with r ∈ Z where all components with negative indices are equal to 0). Let Nr = Im ir (r ∈ N), N = n/n2 , and let S denote the vector σ ∗ -kFurthermore, let Sr (r ∈ N) denote the image of the space (Q + n2 )/n2 ⊆ N .  σ ∗ -k-space (Qr + n2r )/n2r kr k under the map ir . Then k)/ir ( (Qr + n2r )/n2r ) k) Nr = ir ( (nr /n2r ) kr

= nr /(Qr +

n2r )

kr

k=

(mr /m2r )

kr

kr

k,

CHAPTER 4. DIFFERENCE FIELD EXTENSIONS

308

the family (Sr )r∈N is an excellent filtration of the vector σ ∗ -k-space S, and m/m2 = N/S. For every r ∈ N, let (m/m2 )r denote the vector k-subspace of m/m2 generated by the canonical image of mr in m/m2 . Then ((m/m2 )r )r∈N is an excellent filtration of the vector σ ∗ -k-space m/m2 (this is the image of the excellent filtration (Nr )r∈N under the canonical mapping N → m/m2 ). Setting Sr = S ∩ Nr we obtain (see Theorem 3.5.7) that (Sr )r∈N is an excellent filtration of S and, obviously, Nr /Sr = (m/m2 )r for every r ∈ N. By Theorems 3.5.2 and 4.2.5, there exist numerical polynomials ψ1 (t), ψ2 (t), ψ3 (t), and ψ4 (t) in one variable t of degree at most n such that ψ1 (r) = dimk Nr , ψ2 (r) = dimk Sr , ψ3 (r) = dimk Sr and ψ4 (r) = trdegK Ar − trdegK kr for all sufficiently large r ∈ N. Since (Sr )r∈N and (Sr )r∈N are two excellent filtra∆n ψ2 (t) ∆n ψ3 (t) = ∈ Z (as before, for tions of the vector σ ∗ -k-space S, 2n 2n n any polynomial f (t), ∆ f (t) denote the nth finite difference of the polynomial: ∆f (t) = f (t + 1) − f (t), ∆2 f (t) = ∆(∆f (t)), . . . ). Furthermore, Theorem 4.2.5 ∆n ψ4 (t) shows that = σ-trdegK A − σ-trdegK k. Since ψ1 (r) − ψ2 (r) ≥ ψ3 (r) 2n for all sufficiently large r ∈ N (see inequality (4.6.7)), ∆n (ψ1 (t) − ψ3 (t)) ∆n (ψ1 (t) − ψ2 (t)) ∆n ψ4 (t) = ≥ n n 2 2 2n = σ-trdegK A − σ-trdegK k. This completes the proof. σ ∗ -dimk (m/m2 ) =



Suppose that K is a difference (but not necessarily inversive difference) field with a basic set σ and A a local K-algebra with a maximal ideal m. The following example shows that even if m is a difference ideal, it is not necessarily a σ ∗ -ideal of A. Example 4.6.7 Let A be the ring of formal power series in one variable X over Q. Treating Q as an ordinary difference ring whose basic set σ consists of the identical automorphism a σ-Q-algebra ∞ α, wek can consider ∞ A as 2 2k (thus, α( a X ) = α( a X ) for any series by setting α(X) = X k k k=0 k=0 ∞ k a X ∈ A). It is well-known that A is a local Q-algebra whose maximal k k=0 ideal m consists of all power series with zero free terms. Clearly, m is a difference but not an inversive difference ideal of A. Let K be a difference field with a basic set σ = {α1 , . . . , αn } and let R = K{η1 , . . . , ηs } be a σ-K-algebra generated by a finite set η = {η1 , . . . , ηs }. Furthermore, for any k ∈ N, let Rk denote the commutative K-algebra K[{τ (ηi ) | τ ∈ Tσ (k), 1 ≤ i ≤ s}]. Since every Rk is a finitely generated algebra over a field, it has a finite Krull dimension. It would be interesting to describe the function χ(k) = dim Rk , in particular, to determine whether this function is polynomial. Of course, if R is an integral domain, then χ(k) is a polynomial of k, since in this case R is isomorphic to the factor ring of the algebra of σ-polynomials K{y1 , . . . , ys } by a prime σ ∗ -ideal P . Then χ(k) = φP (k) where φP (t) is the σ-dimension polynomial of P (see Definition 4.2.21).

4.6. DIFFERENCE ALGEBRAS

309

If R is not an integral domain, one can consider all essential prime divisors σ-dimension Q1 , . . . , Qm of the perfect ideal {0} of R and the corresponding  polynomials φQi (t) such that φQi (k) = trdegK (Rk /Qi Rk ) (i = 1, . . . , m). By the remark made after Theorem 4.2.15, the set of all difference dimension polynomials is well-ordered with respect to the order ≺ such that f (t) ≺ g(t) if and only if f (r) ≺ g(r) for all sufficiently large r ∈ Z. Let φ(t) = max{φQ1 (t), . . . , φQm (t)} where the maximum is taken with respect to the order ≺. Let φ(t) = φQj (t) for some j, 1 ≤ j ≤ m. It follows from Theorem 1.2.25 that for any k ∈ N, dim Rk = max{trdegK (Rk /Q) | Q is a minimal prime ideal ofRk }.  Since Qj Rk is a prime ideal of Rk , it contains some minimal prime ideal p of Rk (it follows from Proposition 1.2.1(vii) ). Then dim Rk ≥ trdegK (Rk /p) ≥ trdegK (Rk /Qj ), hence χ(k) = dim Rk ≥ φ(k) for all sufficiently large k ∈ N. On the other hand, for any prime ideal Q of Rk (k ∈ N), trdegK (Rk /Q) does

not exceed the number of generators of Rk over k+n (see formula (1.4.16)), we obtain that K. Since this number is s n 

k+n φ(k) ≤ χ(k) ≤ s n for all sufficiently large k ∈ N. Thus, χ(k) = dim Rk is a function of polynomial growth. However, the conditions that imply the polynomial form of the function χ(k) seem to be unknown. Of course, a similar observation can be made in the case of a finitely generated inversive difference algebra over an inversive difference field.

Chapter 5

Compatibility, Replicability, and Monadicity 5.1

Compatible and Incompatible Difference Field Extensions

Definition 5.1.1 Let K be a difference field with a basic set σ and let L and M be two σ-overfields of F . The difference field extensions L/K and M/K are said to be compatible if there exists a σ-field extension N of K such that L/K and M/K have σ-isomorphisms into N/K (i.e., there exist σ-isomorphisms of L and M into N that leave the σ-field K fixed). Otherwise, the σ-field extensions L/K and M/K are called incompatible. The following example is due to R. Cohn. Example 5.1.2 Let us consider Q as an ordinary difference field whose basic set σ consists of the identity automorphism α. If one adjoins to Q an element i such that i2 = −1, then the resulting field Q(i) has two automorphisms that extend α: one of them is the identical mapping (we denote it by the same letter α) and the other (denoted by β) sends an element a + bi ∈ Q(i) (a, b ∈ Q) to a − bi (complex conjugation). Then Q(i) can be treated as a difference field with the basic set {α}, as well as a difference field with the basic set {β}. Denoting these two difference fields by L and M , respectively, we can naturally consider them as σ-field extensions of Q. Let us show that L/Q and M/Q are incompatible σ-field extensions. Indeed, suppose that there is a σ-field extension E of Q and σ-isomorphisms φ and ψ, respectively, of L/Q and M/Q into E/Q. Let j = φ(i), k = ψ(i), and let γ be the translation of E that extends α and β. Then j 2 = k 2 = −1 whence either j = k or j = −k. Since γ(j) = j and γ(k) = −k, in both cases we obtain that j = −j, that is, j = 0. This contradiction (j 2 = −1) implies that the σ-field extensions L/Q and M/Q are incompatible. 311

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The existence of incompatible extensions plays an important part in the development of the theory of difference algebra. Of particular concern here is the fact that the presence of incompatible extensions can inhibit the extension of difference isomorphisms for difference field extensions. Notice that there is no such a phenomenon as incompatibility in the classical field theory: by Theorem 1.6.40(ii), for every two field extensions L and M of a field K, there is a field extension of K which contains K-isomorphic copies of L and M . Exercise 5.1.3 As in the preceding example, let us consider Q as an ordinary difference field with a basic set σ = {α} where α is the identity automorphism. Let p(x) be an irreducible quadratic polynomial with rational coefficients and let a1 and a2 be its roots in C. Let K denote the ordinary difference field Q(a1 , a2 ) with the identical translation, and let L be the difference field Q(a1 , a2 ) with the translation β that maps a1 to a2 and a2 to a1 . Prove that the difference field extensions K/Q and L/Q are incompatible. In what follows we consider some results that play key roles in the study of the phenomenon of incompatibility. The first two theorems are natural generalizations of the results by R. Cohn [41, Chapter 7] to partial difference field extensions. Theorem 5.1.4 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let L1 and L2 be two σ ∗ -field extensions of K. Let M be an overfield of K such that there exist field K-isomorphisms φi of Li into M (i = 1, 2) with the following properties: a) The images φ1 (L1 ) and φ2 (L2 ) are quasi-linearly disjoint over K; b) The compositum of φ1 (L1 ) and φ2 (L2 ) coincides with M . Then there is a unique way of defining the action of elements of σ as injective endomorphisms of M so that M becomes a σ-field extension of K and φi become σ-K-isomorphisms of Li into M (i = 1, 2). PROOF. It is easy to see that for any αj ∈ σ (1 ≤ j ≤ n), φ1 αj (K) and φ2 αj (K) are quasi-linearly disjoint over K (see Theorem 1.6.44(i)). Further(i = 1, 2) is an isomorphism of φi (Li ) onto more, the mapping ψij = φi αj φ−1 i φi (αj (Li )) which coincides with αj on K. By Theorem 1.6.43, there exists a unique extension of ψ1j and ψ2j to an isomorphism of M onto the compositum of φ1 αj (K) and φ2 αj (K). This extension defines an injective endomorphism of M that will be denoted by the same letter αj . Furthermore, our construction of the extensions of the endomorphisms αj of K (1 ≤ j ≤ n) shows that the corresponding endomorphisms of M are pairwise commuting, so M can be treated as a σ-field extension of K. Clearly, φ1 and φ2 become, respectively, σ-K-isomorphisms of L1 and L2 into M . The uniqueness of the desired σ-field structure on M follows from the fact that the action of any αj on M should extend ψ1j and ψ2j , and by Theorem 1.6.3, such an extension is unique. 

5.1. COMPATIBLE AND INCOMPATIBLE EXTENSIONS

313

Corollary 5.1.5 Let K be an inversive difference field with a basic set σ and let L1 and L2 be two σ ∗ -field extensions of K. If the field extension L1 /K is primary, then there exist a σ-field extension M of K and σ-K-isomorphisms φi : Li → M (i = 1, 2) such that φ1 (L1 ) and φ2 (L2 ) are quasi-linearly disjoint over K. PROOF. By Theorems 1.6.40(ii) and 1.6.43, there exist an overfield F of K and K-isomorphisms φi of Li into M (i = 1, 2) such that the fields φ1 (L1 ) and φ2 (L2 ) are quasi-linearly disjoint over K and M is the compositum of these fields. By Theorem 5.1.4, M has a unique structure of a σ-field extension of K with respect to which φ1 and φ2 are σ-K-isomorphisms of L1 and L2 into M , respectively.  Before stating the next theorem we would like to notice that if L/K is a difference field extension with a basic set σ and S(L/K) is the separable closure of K in L (that is, the set of all elements of L which are algebraic and separable over K), then S(L/K)) is an intermediate σ-filed of L/K. Theorem 5.1.6 Let K be a difference field with a basic set σ and let L1 and L2 be two σ-field extensions of K. Furthermore, let S1 and S2 denote the separable closures of K in L1 and L2 , respectively. Then the difference field extensions L1 /K and L2 /K are compaitible if and only if the σ-field extensions S1 /K and S2 /K are compaitible. PROOF. Clearly, if L1 /K and L2 /K are compaitible, so are S1 /K and S2 /K. In order to prove the converse statement, assume first that the σ-field K is inversive and L1 and L2 are σ ∗ -field extensions of K. Then S1 and S2 are also inversive, so they can be also treated as σ ∗ -field extensions of K. Let M be a σ-field extensions of K such that S1 and S2 have σ-K-isomorphisms into M . Without loss of generality we may assume that M actually contains S1 and S2 . By Corollary 5.1.5, the field extensions L1 /S1 and M/S1 are compatible. Let N be a σ-overfield of M such that there exists a σ-S1 -isomorphism φ of L1 into N . Applying Corollary 5.1.5 once again we obtain that N/S2 and L2 /S2 are compatible. Let Q be a σ-overfield of L2 such that there is a σ-S2 isomorphism ψ of N into Q. Then ψφ is a σ-K-isomorphism of L1 into Q. Thus, the σ-field extensions L1 /K and L2 /K are compatible. Now suppose that L1 /K and L2 /K are difference (σ-) but not necessarily inversive difference field extensions. As usual, for every difference (σ-) field F , let F ∗ denote the inversive closure of F . Then the σ ∗ -field extensions S1∗ /K ∗ and S2∗ /K ∗ are compatible. Furthermore, it is easy to see that S1∗ and S2∗ are separable closures of K ∗ in L∗1 and L∗2 , respectively. It follows that the σ ∗ -field extensions L∗1 /K ∗ and L∗2 /K ∗ are compatible hence the σ-field extensions L1 /K  and L2 /K are also compatible. Corollary 5.1.7 Let K be an ordinary difference field with a basic set σ = {α} and let L1 /K and L2 /K be two compatible σ-field extensions of K. Then there

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exist a σ-field extension M of L2 and a σ-K-isomorphism ρ of L1 into M with the following property: If F is any intermediate σ-field of L1 /K, then σ-trdegL2 ρ(F ) L2 ρ(L1 ) = σ-trdegF L1 and Eord L2 ρ(L1 )/L2 ρ(F ) = Eord L1 /F . PROOF. Since passing to inversive closures does not change the difference transcendence degree and effective order of a σ-field extension, we can assume that the σ-fields K, L1 , L2 , and F are inversive. Let S1 and S2 denote the separable closures of K in L1 and L2 , respectively. As in the proof of Theorem 5.1.6, let M be a σ-overfield of K such that S1 and S2 have σ-K-isomorphisms into M . Without loss of generality we may assume that the σ-field M is generated by S1 and S2 ; in particular the field extension M/K is algebraic. Let N be a σ-field extension of M such that there is a σ-S1 -isomorphism φ of L1 into N . Clearly, if a subset A of L1 is algebraically dependent or independent over K, then the set φ(A) is algebraically dependent or independent, respectively, over M and over S2 . By Corollary 5.1.5, one can choose a σ-overfield Q of L2 such that there is a σ-S2 -isomorphism ψ of L2 into Q, and L2 and ψ(N ) are quasi-linearly disjoint over S2 . Therefore, if a set B ⊆ N is algebraically dependent or independent over S2 , the set ψ(B) is algebraically dependent or independent, respectively, over L2 . Let ρ = ψ ◦ φ. Then ρ is a σ-K-isomorphism of L1 into Q, and if a set C ⊆ L1 is algebraically dependent or independent over K, then the set ρ(C) is algebraically dependent or independent, respectively, over L1 . Therefore, if a subset of L1 is algebraically dependent or independent over F , its image under ρ is algebraically dependent or independent, respectively, over L2 (ρ(F )). Since the order (which is also the effective order in the case of inversive fields) and the σ-transcendence degree of a σ-field extension are completely determined by its algebraically independent sets, we obtain the conclusion of the corollary.  Theorem 5.1.8 Let K be an inversive difference field with a basic set σ and let L be a σ-overfield of K such that the field extension L/K is primary. Furthermore, let φ be a σ-isomorphism of K into L. Then there exists an extension of φ to a σ-isomorphism ψ of L into a σ-overfield of M of L such that M = Lψ(L), L and ψ(L) are quasi-linearly disjoint over φ(K), and M/L is a primary field extension. Furthermore, if L is inversive, then M is also inversive. If N is a σ-overfield of L such that φ has an extension to a σ-isomorphism χ of L into N and N is the free join of L and χ(L) over φ(K), then there exists a unique σ-L-isomorphism ρ : M → N such that ρψ(a) = χρ(a) for every a ∈ L. PROOF. It is easy to see that φ(K) is an inversive σ-subfield of L. Let us extend φ to a σ-isomorphism φ of L onto a σ-overfield H of φ(K). Then, by the condition of our theorem, the field extension H/φ(K) is primary. Applying Corollary 5.1.5 we obtain that there exist a σ-field extension M of φ(K) and σ-φ(K)-isomorphisms µ : H → M and ν : L → M such that the field extensions µ(H)/φ(K) and ν(L)/φ(K) are quasi-linearly disjoint. Without loss of generality we may assume that L is a σ-subfield of M and, with ψ denoting the

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composition map µφ, M is the compositum of L and ψ(L). then L and ψ(L) are quasi-linearly disjoint over φ(K). Since M is the free join of L and ψ(L) over φ(K) and the field extension ψ(L)/φ(K) is primary, we can apply Proposition 1.6.49 and obtain that the extension M/L is also primary. Furthermore, it follows from Proposition 2.1.8 that if the σ-field L is inversive, then M = Lψ(L) is inversive as well. Applying Theorem 1.6.40 to the σ-φ(K)-isomorphism χψ −1 of ψ(L) onto χ(L) and the identity automorphism of L, we obtain that there is a unique L-isomorphism ρ of M onto N which coincides with χψ −1 on ψ(L). Since the σ-field structures induced on N by ρ and by the action of σ on L and χ(L) coincide, and since L and χ(L) are quasi-linearly disjoint over φ(K) (by virtue of µ), it follows by Theorem 1.6.43 applied to each αi ∈ σ that the σ-field structure on N is the same as the σ-field structure determined on N by M and ρ. Thus, ρ is a σ-isomorphism of M onto N . The uniqueness of ρ in the sense of the statement of the theorem follows from the uniqueness of ρ in the above sense (as an L-isomorphism M → N which coincides with χψ −1 on ψ(L)).  Exercise 5.1.9 Prove the statement obtained from Theorem 5.1.8 by replacing the terms “primary” and “quasi-linearly disjoint” by “regular” and “linearly disjoint”, respectively. The following theorem is a version of the R. Cohn’s result for ordinary difference field extensions, see [41, Chapter 9, Theorem I]. Theorem 5.1.10 Let K be a difference field with a basic set σ and let L be a σ-overfield of K such that L/K is compatible with every σ-field extension of K. Then every σ-isomorphism φ of K into a σ-overfield M of L can be extended to a σ-isomorphism of L into a σ-overfield of M . PROOF. First of all notice that one can extend φ to a σ-isomorphism of L onto a σ-overfield of φ(K) (this extension will be still denoted by φ). Indeed, all we need to do for constructing such an extension is to choose a subset S of M \ φ(K) whose cardinality is equal to Card(L \ K), fix a bijection ρ : L → φ(K) ∪ S that extends φ, and make L = φ(K) ∪ S a σ-overfield of φ(K) by defining the operations and action of elements α ∈ σ as follows: if x, y ∈ L , then xy = ρ(ρ−1 (x)ρ−1 (y)), x + y = ρ(ρ−1 (x) + ρ−1 (y)), and α(x) = ρ(α(ρ−1 (x))). In what follow we fix such an extension of φ onto an overfield of φ(K) and denote it by the same letter φ. (Of course, φ(L) need not lie in a σ-overfield of M .) Since φ is a σ-isomorphism, the condition of the theorem imply that φ(L)/φ(K) is compatible with every σ-field extension of φ(K). Therefore, there exist a σ-overfield N of M and a σ-φ(K)-isomorphism ψ of φ(L) into N . Obviously, ψφ maps L into N and coincides with φ on the field K.  The next example shows that the requirement of compatibility of L/K with every σ-field extension of K is essential in the statement of Theorem 5.1.10.

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Example 5.1.11 [41, Chapter 9, Example 1]) Let Q be the field of rational numbers treated as an ordinary difference field with the identity translation α. Let a and i denote the positive fourth root of 2 and the square root of −1, respectively (we assume a, i ∈ C). Furthermore, let us consider L = Q(i, a) as a difference overfield of Q where the action of α is defined by the equalities α(i) = −i and α(a) = −a, and let us treat the field K = Q(i) as an intermediate difference field of L/Q. Suppose that φ is a difference K-isomorphism of L into a difference overfield of L. Since the field extension L/K is normal, φ is an automorphism of L (see Theorem 1.6.8). Also, the equalities a4 = 2 = φ(2) = (φ(a))4 imply that φ(a) = aik for some positive integer k. Since α(a) = a, α(aik ) = −aik . On the other hand, α(aik ) = (−i)k (−a) = (−1)k+1 (aik ). It follows that k is even, hence φ(a2 ) = a2 . Thus, every difference K-isomorphism of L leaves fixed elements of the field Ka2 . At the same time, it is easy to see that there is a difference Kautomorphism ψ of Ka2  such that ψ(a2 ) = −a2 . Obviously, ψ has no extension to a difference K-isomorphism of L. The following statement describes a particular class of ordinary difference field extensions L/K that are compatible with every difference field extension of K. Proposition 5.1.12 Let K be an ordinary difference field with a basic set σ = {α}, K{y1 , . . . , ys } the algebra of σ-polynomials in σ-indetermiantes y1 , . . . , ys over K and Φ a family of linear homogeneous σ-polynomials in K{y1 , . . . , ys }. Then (i) The reflexive closure P of the σ-ideal [Φ] is a prime reflexive σ-ideal. (ii) If η = (η1 , . . . , ηs ) is a generic zero of P , then the field L = Kη1 , . . . , ηs  is purely transcendental over K (in the usual sense of the field theory). (iii) The difference field extension L/K is compatible with every σ-field extension of K. PROOF. Let K ∗ denote the inversive closure of K and let R denote the algebra of σ ∗ -polynomials in σ ∗ -indeterminates y1 , . . . , ys over K ∗ (thus, R can be treated as an inversive closure of the σ-ring K{y1 , . . . , ys }). Let Φ∗ = {αi f | i ∈ Z, f ∈ Φ} ⊆ R and let Ψ denote the set of all linear combinations of elefor every k = 0, 1, . . . , let ments of Φ∗ with coefficients in K ∗ . Furthermore,  Yk = {αi yj | 0 ≤ i ≤ k, 1 ≤ j ≤ s} and Ψk = Ψ K[Yk ]. Then Ψk is a system of linear polynomials in the polynomial ring K[Yk ]. Clearly, this system is closed under linear combinations with coefficients in K and generates a prime ideal in  K[Yk ]. Furthermore, if m, n ∈ N and n > m, then Ψn K[Ym ] = Ψm and every solution of Ψm as a set of linear polynomials inK[Ym ] extends to a solution of Ψn . Then a generic zero of the ideal Pm = P K[Ym ] in K[Ym ] extends to a solution of Φn and hence of Pn . Therefore, Pm = Pn K[Ym ]. Furthermore, denoting the set {αi yj | 1 ≤ i ≤ k, 1 ≤ j ≤ s} (k ≥ 1) by Yk and using the abovearguments one obtains that Pn ∩ K[Yn ] is the ideal generated in K[Yn ] by Ψ K[Yn ]. Since for any σ-polynomials f , the inclusion α(f ) ∈ Ψ implies f ∈ Ψ, we obtain that the inclusion α(f ) ∈ Pn implies f ∈ Pn .

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∞  It follows from the above discussion that P = Pk is a reflexive prime k=0  σ-ideal in K{y1 , . . . , ys } and P K[Yk ] = Pk for all k ∈ N. Therefore, P is the reflexive closure of [Φ]. In order to prove (ii), we notice that if η = (η1 , . . . , ηs ) is a generic zero of P , then the sk-tuple (η1 , . . . , ηs , αη1 , . . . , α1 ηs , . . . , αk ηs ) is a generic zero of Pk . It follows that K(η1 , . . . , ηs , αη1 , . . . , α1 ηs , . . . , αk ηs ) is purely transcendental over K for k = 0, 1. . . . whence L = Kη1 , . . . , ηs  is purely transcendental over K. Statement (iii) is a direct consequence of Theorem 5.1.6. 

Let K be a difference field with a basic set σ = {α1 , . . . , αn }. In what follows, for any αi1 , . . . , αik ∈ σ (1 ≤ k ≤ n), (K; αi1 , . . . , αik ) will denote the field K treated as a difference field with a basic set {αi1 , . . . , αik }. Definition 5.1.13 With the above notation, we say that K satisfies the universal compatibility condition if every two σ-extensions of K are compatible. K is said to satisfy the stepwise compatibility condition if there exists a permutation (i1 , . . . , in ) of (1, . . . , n) such that all difference fields (K; αi1 , . . . , αik ), 1 ≤ k ≤ n satisfy the universal compatibility condition. Remark 5.1.14 It follows from Theorem 5.1.6 that any algebraically closed or separably algebraically closed difference field, as well as the inversive closure of such a field (see Proposition 2.1.9(i) ), satisfies the stepwise compatibility condition. Theorem 5.1.15 Let K be a difference field with a basic set σ, α ∈ σ, and Kα denote the field K treated as a difference field with the basic set σ \ {α}. If for every α ∈ σ, Kα satisfies the stepwise compatibility condition, then there exists a σ-overfield L of K such that L is an algebraic closure of K. PROOF. We proceed by induction on n = Card σ. Suppose first that n = 1, so that K is an ordinary difference field with a basic set σ = {α}. Applying Theorem 1.6.6(v) with L = K, M = K (an algebraic closure of K) and φ = i◦α, where i is the embedding of K into K, we obtain that there exists an extension of α to an endomorphism of K. Thus, K can be treated as a σ-overfield of K. Assume the statement of the theorem to be true for n = q where q is a positive integer. Let K be a difference field with a basic set σ = {α1 , . . . , αq+1 } consisting of q + 1 translations such that, for some ordering αi1 , . . . , αiq+1 of elements of σ, the difference field K  = (K; αi1 , . . . , αiq ) satisfies the stepwise compatibility condition. Of course, K  satisfies the condition on K of the statement of the theorem, so we can apply the induction hypothesis and, setting σ  = {αi1 , . . . , αiq }, obtain that there exists an algebraic closure H of K that has a structure of a σ  -overfield of K  . Since any two σ  -field extensions of K  are compatible, we may, by Theorem 5.1.10, extend αiq+1 , considered as a σ  isomorphism of K  into K  , to a σ  -isomorphism αiq+1 of H into a σ  -overfield of H. For each element a ∈ H, αiq+1 (a) is algebraic over the field αiq+1 (K  ) and hence also over K. Therefore (since K admits at most one algebraic closure

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in any given overfield of K), αiq+1 (a) ∈ H. It follows that the structure of H as a σ  -field, together with αiq+1 (K  ), determines a σ-overfield structure on an algebraic closure of K (the action of αq+1 is defined as the action of αiq+1 ). Thus, the proof is completed by induction.  The first part of the proof of Theorem 5.1.15 leads to the following statement about ordinary difference fields. Corollary 5.1.16 Let K be an ordinary difference field with a basic set σ = {α}. Then there exists an algebraic closure K of K that has a structure of a σ-overfield of K.  Although any ordinary difference field K has a difference overfield which is an algebraic closure of K, the following example provided by I. Bentsen [10] shows that this result cannot be generalized to partial difference fields. Example 5.1.17 Let us consider Q as a difference field with a basic set σ = {α1 , α2 } where α1 and α2 are the identity automorphisms. Let b and i denote the positive square root of 2 and the square root of −1, respectively (we assume b, i ∈ C), and let F = Q(b, i). Let us extend α1 and α2 to automorphisms of the field F setting α1 (b) = −b, α1 (i) = i, α2 (b) = b, α2 (i) = −i, and consider F as a σ-overfield of Q. Then there exists no σ-overfield of F which is an algebraic closure of F . Indeed, if it is not so, then there exists a σ-overfield G of F which contains an element a such that a2 = b. Since (α1 (a))2 = α1 (a2 ) = −b and (α2 (a))2 = α2 (a2 ) = b, we have α1 (a) = λai and α2 (a) = µa where λ and µ denote plus or minus 1. Then α1 α2 (a) = λµia and α2 α1 (a) = −λµia. Thus, α1 and α2 do not commute at a, which contradicts the assumption that G is a σ-overfield of F . The following example, also due to I. Bensten [10], shows that the converse of Theorem 5.1.15 is not true. Example 5.1.18 Let Q be the field of rational numbers treated as a difference field with two translations α1 and α2 each of which acts as an identity automorphism on Q. α1 and α2 extend trivially to identity automorphisms of the algebraic closure H of Q contained in C. Let K denote Q treated as an ordinary difference field with a basic set σ  = {α} where α is either α1 or α2 , and let F = Q(i) (i2 = −1). Let L1 and L2 denote the σ  -field extensions of K constructed on F via the continuations of α such that α(i) = i and α(i) = −i, respectively. (As usual, we denote the continuation by the same letter α.) Then the σ  -field extensions L1 /K and L2 /K are incompatible (see Example 5.1.2), so for either definition of K, K does not satisfy the universal compatibility condition.

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Difference Kernels over Ordinary Difference Fields

In this section we consider R. Cohn’s theory of difference kernels over ordinary difference fields. This theory plays an important role in the theory of ordinary algebraic difference equations; in particular, the technique of difference kernels, as we shall see in Chapter 7, allows one to prove the fundamental existence theorem for ordinary difference polynomials. Below we basically follow [41]; the corresponding theory for partial difference fields is presented in Chapter 6. Definition 5.2.1 Let K be an ordinary inversive difference field with a basic set σ = {α}. A difference (or σ-) kernel of length r ≥ 1 over K is an ordered (1) (s) pair R = (K(a0 , . . . , ar ), τ ) where each ai is itself an s-tuple (ai , . . . , ai ) over K (a positive integer s is fixed) and τ is an extension of α to an isomorphism of K(a0 , . . . , ar−1 ) onto K(a1 , . . . , ar ) such that τ ai = ai+1 for i = 0, . . . , r − 1. j) (j) (In other words, τ (ai ) = ai+1 for 0 ≤ i ≤ r − 1, 1 ≤ j ≤ s.) If r = 0, then R = (K(a0 ), τ ) where τ = α : K → K. Two kernels R = (K(a0 , . . . , ar ), τ ) and R = (K(b0 , . . . , bq ), τ  ) (where b0 has the same number of coordinates as a0 ) are said to be equivalent (or isomorphic) if q = r and there exists a K-isomorphism φ : K(a0 , . . . , ar ) → K(b0 , . . . , br ) such that φ(ai ) = bi (i = 0, . . . , r) φτ = τ  φ on K(a0 , . . . , ar−1 ). Note, that in the last definition and below we use the following convention: if elements x1 , . . . , xm belong to the domain of a mapping φ and x denotes the m-tuple (x1 , . . . , xm ), then φ(x) is the m-tuple (φ(x1 ), . . . , φ(xm )). Definition 5.2.2 With the above notation, the transcendence degree of the difference kernel R is defined to be trdegK(a0 ,...,ar−1 ) K(a0 , . . . , ar ); it is denoted by δR. In what follows, while considering difference kernels over an ordinary difference field K, we always assume that K is inversive. Definition 5.2.3 Let R = (K(a0 , . . . , ar ), τ ) be a difference kernel of length r over an ordinary inversive difference field K. A prolongation R of R is defined as a difference kernel of length r+1 consisting of an overfield K(a0 , . . . , ar , ar+1 ) of K(a0 , . . . , ar ) and an extension τ  of τ to an isomorphism of K(a0 , . . . , ar ) onto K(a1 , . . . , ar+1 ). As before, a set of the form a = {a(i) |i ∈ I} will be referred to as an indexing a (with the index set I), and if J ⊆ I, then the set {a(i) |i ∈ J} will be called ˜0 is a a subidexing of a. If R = (K(a0 , . . . , ar ), τ ) is a difference kernel and a (i ) (i ) subindexing of a0 (that is, a ˜0 = (a0 1 , . . . , a0 q ), 1 ≤ i1 < · · · < iq ≤ s), then a ˜k (k = 1, . . . ) will denote the corresponding subindexing of ak . Theorem 5.2.4 Every difference kernel R = (K(a0 , . . . , ar ), τ ) over an ordinary difference field K with a basic set σ = {α} has a prolongation R =

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(K(a0 , . . . , ar , ar+1 ), τ  ). Moreover, one can chose a prolongation R with the following properties. ˜r is algebraically independent over (i) If a ˜0 is a subindexing of a0 such that a ˜r+1 is algebraically independent over K(a0 , . . . , ar ) (so K(a0 , . . . , ar−1 ), then a that δR = δR ). r+1  a ˜i is algebraically independent over K. (ii) The set i=0

PROOF. Let P be the prime ideal with generic zero ar of the polynomial ring K(a0 , . . . , ar−1 )[X1 , . . . , Xs ] in indeterminates X1 , . . . , Xs . Let P  be the set obtained from P by replacing the coefficients of the polynomials of P by their images under τ . Clearly, P  is a prime ideal of the polynomial ring K(a1 , . . . , ar )[X1 , . . . , Xs ]. Let J denote the ideal of the polynomial ring K(a0 , a1 , . . . , ar )[X1 , . . . , Xs ] generated by P  , let Q be an essential prime divisor of the ideal J in , and let ar+1 be a generic zero of Q. Since ar+1 is also a generic zero of P  , Theorem 1.2.42(iv) implies that there is an isomorphism τ  of K(a0 , . . . , ar ) onto K(a1 , . . . , ar+1 ) which extends τ . Thus, R = (K(a0 , . . . , ar+1 ), τ  ) is a prolongation of R. ˜r is algebraically independent over Let a ˜0 be a subindexing of a0 such that a ˜r is contained in a transcendence basis br of ar over K(a0 , . . . , ar−1 ). Then a K(a0 , . . . , ar−1 ). Then the existence of the isomorphism τ  implies that br+1 is a transcendence basis of ar+1 over K(a1 , . . . , ar ). By Theorem 1.2.42(viii), every complete set of parameters of P  is a complete set of parameters of Q, hence br+1 is also a transcendence basis of ar+1 over K(a0 , . . . , ar ). It follows that br+1 and therefore its subset a ˜r+1 are algebraically independent over K(a0 , . . . , ar ). To prove statement (ii) note that for every i = 0, . . . , r +1, a ˜i is algebraically independent over K(a0 , . . . , ai−1 ). Indeed, we have proved this for i = r + 1, it is true for i = r by the condition of the theorem, and for every i = 0, . . . , r − 1 the statement follows from the fact that τ r−i is an isomorphism of K(a0 , . . . , ai ) onto ˜i is algebraically independent over K(˜ a0 , . . . , a ˜i−1 ) K(ar−i , . . . , ar ). Therefore, a r+1  a ˜i is algebraically independent over K.  whence the set i=0

Definition 5.2.5 Let R = (K(a0 , . . . , ar ), τ ) be a difference kernel of length r over an ordinary inversive difference field K. A prolongation R of the kernel R is called generic if δR = δR. A generic prolongation of a difference kernel R = (K(a0 , . . . , ar ), τ ) over an inversive ordinary difference field can be constructed as in the proof of Theorem 5.1.4. Indeed, let P be the prime ideal with generic zero ar of a polynomial ring K(a0 , . . . , ar−1 )[X1 , . . . , Xs ] in s indeterminates X1 , . . . , Xs . Let P  be obtained from P by replacing the coefficients of the polynomials of P by their images under τ . Then P  is a prime ideal of K(a1 , . . . , ar )[X1 , . . . , Xs ] and generates an ideal Pˆ in K(a0 , . . . , ar )[X1 , . . . , Xs ]. Let Q be an essential prime divisor of Pˆ in the last ring and let ar+1 be a generic zero of Q. Then ar+1 is also a generic

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zero of P  and there is an isomorphism τ  : K(a0 , . . . , ar ) → F (a1 , . . . , ar+1 ) that extends τ . We obtain the desired generic prolongation. Conversely, if R = (K(a0 , . . . , ar+1 ), τ  ) is any generic prolongation of R, then ar+1 is a solution of the ideal Pˆ and hence of one of its essential prime divisors Q. It follows that coht Q = coht P = trdegK(a0 ,...,ar−1 ) K(a0 , . . . , ar ) = trdegK(a0 ,...,ar ) K(a0 , . . . , ar+1 ), so that ar+1 is a generic zero of Q. Thus, every generic prolongation of R can be constructed by the foregoing procedure. Example 5.2.6 (see [41, Chapter 6, section 2]). Let K be an inversive ordinary difference field of zero characteristic with a basic set σ = {α}, K{y} the ring of σ-polynomials in one σ-indeterminate y over K, and A an irreducible σ-polynomial in K{y} (that is, A is irreducible as a polynomial in variables y, αy, α2 y, . . . over K). Assuming that A contains y and αm y, m > 0, is the highest transform of y in A, we shall use prolongations of difference kernels to construct a solution of A. First, let us consider an m-tuple a = (a(1) , . . . , a(m) ) whose coordinates constitute an algebraically independent set over K. Now we (i) define an m-tuple a1 as follows: we set a1 = a(i+1) for i = 1, . . . , m − 1, replace (i) yi−1 by a in A (1 ≤ i ≤ m), find a solution of the resulting polynomial in one unknown ym and take it as a(m) . Since A involves y, a(1) will be algebraically de(m) (1) (m) pendent on a(2) , . . . , a(m) , a1 over K. Therefore, a1 , . . . , a1 are algebraically independent over K whence there is a difference kernel R1 defined over K by the extension of the translation α to an isomorphism τ0 : K(a) → K(a1 ). By successive application of Theorem 5.2.4 we find a sequence a0 = a, a1 , . . . such that a kernel Rk+1 is defined by an isomorphism τk : K(a0 , . . . , ak ) → K(a1 , . . . , ak+1 ) (k = 0, 1, . . . ) and Rk+1 is a prolongation of Rk . Then K(a0 , a1 , . . . ) becomes a difference overfield of K where the extension of α (denoted by the same letter) is defined by α(b) = τk (b) whenever b ∈ K(a0 , . . . , ak ). It is clear that this field coincides with Ka(1)  and the element a(1) is a solution of A. Theorem 5.2.7 Let R = (K(a0 , . . . , ar ), τ ) be a difference kernel over an inversive ordinary difference field K with a basic set σ = {α}. (i) There are only finitely many distinct (that is, pairwise non-isomorphic) generic prolongations of R. (ii) Let R = (K(a0 , . . . , ar+1 ), τ  ) be a generic prolongation of a R and let a ˜0 be a subindexing of a0 which is algebraically independent over K(a1 , . . . , ar ). Then a ˜0 is algebraically independent over K(a1 , . . . , ar+1 ). PROOF. Let Q1 , . . . , Qk be all essential prime divisors of the ideal J defined in the proof of Theorem 5.2.4. As we have mentioned in the discussion after Definition 5.2.5, every Qi yields a generic prolongation R of R and all generic prolongation can be obtained in this way. To prove the second part of the theorem, one may assume that a ˜0 is a σ-transcendence basis of a0 over K(a1 , . . . , ar ) (that is, a transcendence basis of K(a0 , . . . , ar ) over K(a1 , . . . , ar )). Since τ  is an isomorphism of K(a0 , . . . , ar ) onto K(a1 , . . . , ar+1 ), trdegK K(a0 , . . . , ar−1 ) = trdegK K(a1 , . . . , ar ) and trdegK K(a0 , . . . , ar ) = trdegK K(a1 , . . . , ar+1 ). Applying the additive property of transcendence degree (see Theorem 1.6.30(iv))

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we obtain trdegK(a1 ,...,ar+1 ) K(a0 , . . . , ar+1 ) = trdegK(a0 ,...,ar ) K(a0 , . . . , ar+1 ) = trdegK(a0 ,...,ar−1 ) K(a0 , . . . , ar ) = trdegK(a1 ,...,ar ) K(a0 , . . . , ar ). Since a ˜0 contains a transcendence basis of a0 over K(a1 , . . . , ar+1 ), the last ˜0 is algebraically independent equalities show that a ˜0 is such a basis, hence a  over K(a1 , . . . , ar+1 ). Let R = (K(a0 , . . . , ar ), τ ) be a difference kernel over an inversive ordinary difference field K with a basic set σ = {α} and let b0 be a subindexing of a0 . If br is a transcendence basis of ar over K(a0 , . . . , ar−1 ), then b0 is called a special set. Clearly, such a set consists of δR elements and it is also a special set for any generic prolongation R of R. If b0 is a subindexing of a0 such that b0 contains a special set and trdegK(b0 ,...,br−1 ) K(b0 , . . . , br ) = δR, then the order of R with respect to b0 is defined as ordb0 R = trdegK(b0 ,...,br ) K(a0 , . . . , ar ). (In this case, b0 is said to be a subindexing of a0 for which ordb0 R is defined.) Suppose that b0 itself is a special set. Then it is easy to see that ordb0 R is defined and ordb0 R = trdegK(b0 ,...,br ) K(a0 , . . . , ar ) = trdegK(b0 ,...,br ) K(a0 , . . . , ar−1 , br ) = trdegK(b0 ,...,br−1 ) K(a0 , . . . , ar−1 ), see Theorem 1.6.30(iv) . If δR = 0, we consider b0 = ∅ and define the order ord R of the kernel R as ord R = trdegK K(a0 , . . . , ar ) = trdegK K(a0 , . . . , ar−1 ). If r = 0, then ordb0 R is defined if b0 contains a special set which is a transcendence basis of K(a0 ) over K. In this case ordb0 R = 0. If a subindexing b0 of a0 is a special set, we define the degree db0 R and reduced degree rdb0 R of R with respect to b0 to be K(a0 , . . . , ar ) : K(a0 , . . . , ar−1 ; br ) and [K(a0 , . . . , ar ) : K(a0 , . . . , ar−1 ; br )]s , respectively. (As usual, if L/K is a field extension, then [L : K]s denotes the separable degree of L over K.) Theorem 5.2.8 With the above notation, let b0 be a subindexing of a0 for which ordb0 R is defined. (i) If R = (K(a0 , . . . , ar+1 ), τ  ) is a generic prolongation of a kernel R = (K(a0 , . . . , ar ), τ ), then ordb0 R is defined and ordb0 R = ordb0 R . (ii) Suppose that b0 itself is a special set. Then db0 R and rdb0 R are finite. Furthermore, if R1 , . . . , Rh are all distinct (pairwise non-isomorphic) prolongah h tions of R, then i=1 rdb0 Ri = rdb0 R and i=1 db0 Ri ≤ db0 R. (Of course, in the case of characteristic 0 the last inequality becomes an equality.) PROOF. (i) Since ordb0 R is defined, b0 contains a special set which, as we have noticed, is also a special set for the generic prolongation R . Since every transcendence basis of br+1 over K(a0 , . . . , ar ) is algebraically independent over K(b0 , . . . , br ), we obtain that trdegK(b0 ,...,br ) K(b0 , . . . , br+1 ) ≥ trdegK(a0 ,...,ar ) K(a0 , . . . , ar , br+1 ) (5.2.1) = trdegK(a0 ,...,ar ) K(a0 , . . . , ar+1 ) = δR . Applying the isomorphism τ  we obtain that trdegK(b0 ,...,br ) K(b0 , . . . , br+1 ) ≤ trdegK(b1 ,...,br ) K(b1 , . . . , br+1 ) = trdegK(b0 ,...,br−1 ) K(b0 , . . . , br ) = δR .

(5.2.2)

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Since δR = δR (R is a generic prolongation of R), the equalities (5.2.1) and (5.2.2) imply that trdegK(b0 ,...,br ) K(b0 , . . . , br+1 ) = δR .

(5.2.3)

Thus, ordb0 R is defined and, moreover, equations (5.2.1) and (5.2.3) show that trdegK(b0 ,...,br ) K(b0 , . . . , br+1 ) = trdegK(a0 ,...,ar ) K(a0 , . . . , ar+1 ) . whence trdegK K(a0 , . . . , ar ) = trdegK K(b0 , . . . , br ) + trdegK(b0 ,...,br ) K(b0 , . . . , br+1 ) = trdegK K(b0 , . . . , br ) + ordb0 R. The last two equalities imply that trdegK K(a0 , . . . , ar+1 ) = trdegK(a0 ,...,ar ) K(a0 , . . . , ar+1 )+trdegK K(a0 , . . . , ar ) = trdegK(b0 ,...,br ) K(b0 , . . . , br+1 )+trdegK K(b0 , . . . , br ) +ordb0 R = trdegK K(b0 , . . . , br+1 ) + ordb0 R. On the other hand, by the definition of the order we have trdegK K(a0 , . . . , ar+1 ) = trdegK K(b0 , . . . , br+1 ) + ordb0 R whence ordb0 R = ordb0 R . (ii) In the prove the second part of the theorem we use the notation of the proof of Theorem 5.2.4: P denotes the prime ideal of the polynomial ring K(a0 , . . . , ar−1 )[X1 , . . . , Xs ] with generic zero ar , P  is the prime ideal of the ring K(a1 , . . . , ar )[X1 , . . . , Xs ] obtained from P by replacing the coefficients of the polynomials in P by their images under τ , and J is the ideal of K(a0 , a1 , . . . , ar )[X1 , . . . , Xs ] generated by P  . Let Q1 , . . . , Qm be the essential prime divisors of the ideal J. Then each Qi produces a generic prolongation Ri of R, and every generic prolongation of R is isomorphic to one of Ri (see the proof of Theorem 5.2.4 and the remark after Definition 5.2.5). Since b0 is a special set for each Ri , db0 Ri and rdb0 Ri (1 ≤ i ≤ m) are defined. Let u0 be a generic zero of the ideal P  of K(a1 , . . . , ar )[X1 , . . . , Xs ] and let v0 be a the subindexing of u0 with the same superscripts as the coordinates of b0 . Let z1 , . . . , zk be the complete set of conjugates of u0 over K(a1 , . . . , ar , v0 ) (see the corresponding definition after Theorem 1.6.23). Then each Qj (1 ≤ j ≤ m) has some zij as a generic zero. Let c1 , . . . , cm be all such generic zeros. Of course, they form a maximal subset of {z1 , . . . , zk } pairwise not equivalent under a K(a0 , . . . , ar , v0 )-isomorphism. Since db0 R = K(a0 , . . . , ar ) : K(a0 , . . . , ar−1 , br ) = K(a1 , . . . , ar , u0 ) : K(a1 , . . . , ar−1 , v0 ), db0 R = K(a0 , . . . , ar , ci ) : K(a0 , . . . , ar , v0 ) (1 ≤ i ≤ m) and the corresponding equalities hold for the reduced degrees, statement (ii) of the theorem follows from equalities (1.6.1) - (1.6.3). 

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Let K be an ordinary difference field with a basic set σ = {α} and L = Kη1 , . . . , ηs  a σ-overfield of K generated by an s-tuple η = (η1 , . . . , ηs ). Then the contraction of α to an isomorphism τr : K(η, α(η), . . . , αr−1 (η)) → K(α(η), . . . , αr (η)) (r = 1, 2, . . . ) defines a difference kernel R = (K(a0 , . . . , ar ), τr ) of length r over K. Conversely, starting with a difference kernel one can introduce the concept of its realization as follows. Definition 5.2.9 With the above notation, let R = (K(a0 , . . . , ar ), τ ) be a dif(1) (s) ference kernel with a0 = (a0 , . . . , a0 ). An s-tuple η = (η1 , . . . , ηs ) with coordinates from a σ-overfield of K is called a realization of R if η, α(η), . . . , αr (η) is a specialization of a0 , . . . , ar over K. If this specialization is generic, the realization is called regular. If there exists a sequence R(0) = R, R(1) , R(2) , . . . of kernels, each a generic prolongation of the preceding, such that η is a regular realization of each R(i) , then η is called a principal realization of R. Theorem 5.2.10 Let R = (K(a0 , . . . , ar ), τ ) be a difference kernel over an inversive ordinary difference field K with a basic set σ = {α}. (i) There exists a principal realization of the kernel R. If η is such a realization, then σ-trdegK Kη = δR. (ii) Let b0 be a subindexing of a0 such that ordb0 R is defined and let ζ is the corresponding subindexing of a principal realization η of R. Then trdegK ζ Kη = ordb0 R. Furthermore, if b0 is a special set, then ζ is a σ-transcendence basis of Kη over K. (iii) The number of distinct principal realizations of the kernel R is finite. Let us denote them by (1) η, . . .(m) η. If b0 is a special set and (i) ζ is the corresponding m  rld(K(i) η/K(i) ζ) = subset of the components of (i) η (1 ≤ i ≤ m), then rdb0 R and

m 

i=1

ld(K(i) η/K(i) ζ) ≤ db0 R.

i=1

(iv) If a subindexing b0 of a0 is a special set for the kernel R and ζ is the corresponding subindexing of a principal realization η of R, then trdegK ζ Kη = trdeg(K ζ)∗ (Kη)∗ if and only if b0 is algebraically independent over the field K(a0 , . . . , ar ) or b0 = ∅. (v) If η is a regular realization of the kernel R, but not a principal realization, then σ-trdegK Kη < δR. (vi) A realization of the kernel R which specializes over K to a principal realization is a principal realization, and the specialization is generic. PROOF. Repeating the proof of Theorem 5.2.4 we find a sequence of difference kernels R = R0 , R1 , R2 , . . . such that each kernel Rm = (K(a0 , . . . , ar+m ),

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τm ) is a generic prolongation of Rm−1 (m = 1, 2, . . . ). The isomorphisms τm of K(a0 , . . . , ar+m−1 ) onto K(a1 , . . . , ar+m ) define the automorphism of the field K(a0 , a1 , . . . ) which coincides with τm on every its subfield K(a0 , . . . , ar+m ) (m ∈ N). Denoting this automorphism by α, we obtain a structure of the difference field extension Ka0  on the field K(a0 , a1 , . . . ), and a0 is a principal realization of R. Furthermore, as it follows from the remark after Definition 5.2.5, every distinct principal realizations of R are obtained in this way (by choosing all distinct principal prolongations Rm of Rm−1 (m = 1, 2, . . . ) in the above sequence). Let b0 be a subindexing of a0 such that ordb0 R is defined. Since b0 contains a special set, trdegK(a0 ,...,ar+m ,br+m+1 ,...,br+m+k ) K(a0 , . . . , ar+m+k ) = 0 for all m, k ∈ N. By Theorem 5.2.8(i), trdegK(b0 ,...,br+m+k ) K(a0 , . . . , ar+m+k ) = ordb0 R whence trdegK(b0 ,...,br + m + k ) K(a0 , . . . , ar + m , br + m + 1 , . . . , br + m + k ) = ordb0 R. It follows that a transcendence basis of the coordinates of a0 , . . . , ar+m over K(b0 , . . . , br+m ) remains algebraically independent over K(b0 , . . . , br+m+k ) for every k ∈ N, hence it remains algebraically independent over K(b0 , b1 , b2 , . . . ). Thus, trdegK(b0 ,b1 ,b2 ,... ) K(a0 , . . . , ar+m+k , b0 , b1 , b2 , . . . ) = ordb0 R. Since the last equality is true for every m ∈ N, we obtain the equality trdegK b0  Ka0  = ordb0 R which implies that b0 contains a σ-transcendence basis of Ka0  over K. If b0 is a special set, such a basis is a
σ-transcendence basis m of a over K. Indeed, Theorem 5.2.4(ii) shows that the set i=1 bi is algebraically independent over K for every m ∈ N, so that b0 is σ-algebraically independent over K. Since a special set contains δR elements, σ-trdegK Ka = δR. Since all principal realizations can be obtained in the process used to obtain a0 , this completes the proof of the first two statements of the theorem. In order to prove statement (iii) notice that for any special set b0 and any m ∈ N we have db0 Rm+1 ≤ db0 Rm and rdb0 Rm+1 ≤ rdb0 Rm (see theorem 6.1.8(ii)). Therefore, there exists p ∈ N such that db0 Rm = db0 Rp and rdb0 Rm = rdb0 Rp for all integer m ≥ p. It follows that for all such m we have K(a0 , . . . , ar+m ) : K(a0 , . . . , ar+m−1 , br+m ) = db0 Rp . By Theorem 5.2.4, each set br+m+i (i =
1, 2, . . . ) is algebraically independent ∞ over K(a0 , . . . , ar+m+i−1 ), hence the set i=1 br+m+i is algebraically independent over K(a
0 , . . . , ar+m ). ∞ Let U = j=0 bj . Applying Theorem 1.6.28(v) we obtain that K(a0 , . . . , ar+m , U ) : K(a0 , . . . , ar+m−1 , U ) = db0 Rp for all m ≥ p, whence ld(Ka0 / Kb0 ) = db0 Rp . Similarly one can obtain that rld(Ka0 /Kb0 ) = rdb0 Rp . Theorem 5.2.8(ii) shows that the sum of all reduced degrees with respect to a special set b0 of all distinct kernels obtained from R by m successive prolongations is rdb0 R and the sum of their degrees is at most db0 R. This implies the inequality and equality in part (iii) of the theorem. Let us prove statement (iv). If trdegK ζ Kη = trdegK ζ∗ Kη∗ and b0 = ∅, then one has trdegK(ζ,α(ζ),α2 (ζ),... ) K(ζ, α(η), α2 (η), . . . ) = trdegK(ζ,α(ζ),α2 (ζ),... ) K(η, α(η), α2 (η), . . . ) = trdegK(α(ζ),α2 (ζ),... ) K(α(η), α2 (η), . . . ).

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(The last equality is obtained from the second one by applying α to the fields K(ζ, α(ζ), α2 (ζ), . . . ) and K(η, α(η), α2 (η), . . . )). It follows that trdegK(α(η),α2 (η),... ) K(ζ, α(η), α2 (η), . . . ) = trdegK(α(ζ),α2 (ζ),... ) K(ζ, α(ζ), α2 (ζ), . . . ). Since ζ is algebraically independent over the field K(α(ζ), α2 (ζ), . . . ), ζ is also algebraically independent over this field, hence b0 is algebraically independent over K(a0 , . . . , ar ). Conversely, suppose that b0 is algebraically independent over K(a1 , . . . , ar ) or b0 = ∅. Since b0 is a special set, we have trdegK(b0 ,...,br ) K(a0 , . . . , ar ) = trdegK(b0 ,...,br−1 ) K(a0 , . . . , ar−1 ) = trdegK(b1 ,...,br ) K(a1 , . . . , ar ) hence trdegK(a1 ,...,ar ) K(a0 , . . . , ar ) = trdegK(b1 ,...,br ) K(b0 , . . . , br ). At the same time, our assumptions about b0 imply that trdegK(a1 ,...,ar ) K(b0 , a1 , . . . , ar ) = trdegK(b1 ,...,br ) K(b0 , . . . , br ) hence trdegK(a1 ,...,ar ) K(b0 , a1 , . . . , ar ) = trdegK(a1 ,...,ar ) K(a0 , . . . , ar ). The last equality shows that every coordinate of a0 is algebraic over the field K(b0 , a1 , . . . , ar ). Furthermore, using induction on k ∈ N, one can easily obtain that every coordinate of η, α(η), . . . , αk (η) is algebraic over the field K(ζ, α(ζ), . . . , αk (ζ), αk+1 (η), . . . , αk+r (η)) and hence over the field K(ζ, α(ζ), . . . , αk (ζ), αk+1 (η), αk+2 (η), . . . ). We arrive at the equality trdegK(ζ,α(ζ),... ) K(η, α(η), . . . ) = trdegK(ζ,α(ζ),... ) K(ζ, α(ζ), . . . , αk (ζ), αk+1 (η), αk+2 (η), . . . ) which, together with Proposition 4.1.19, implies statement (iv) of our theorem. Let η = (η1 , . . . , ηs ) be a regular realization of R but not a principal realization of this kernel. Let p = σ-trdegK Kη and let ηi1 , . . . , ηip (1 ≤ i1 < · · · < ip ≤ s) be a σ-transcendence basis of Kη over K. Then for every q ∈ N, the set {αj (ηiν ) | 0 ≤ j ≤ q, 1 ≤ ν ≤ p} is algebraically independent over K hence trdegK K(η, α(η), . . . , αq (η)) ≥ p(q + 1).

(5.2.4)

Since η is not a principal realization of R, there exists the smallest integer d, d ≥ r, such that the kernel K(η, α(η), . . . , αd+1 (η)), τd+1 ) is not a generic prolongation of the kernel K(η, α(η), . . . , αd (η)), τd )(we use the notation introduced before Definition 5.2.9). Then αd+1 (η) is a zero but not a generic zero of an ideal of dimension δR similar to the ideal Q in the proof of Theorem 5.2.4. Therefore, trdegK(η,α(η),...,αd (η)) K(η, α(η), . . . , αd+1 (η)) ≤ δR − 1. It is easy to see that for every m ∈ N, trdegK(η,α(η),...,αd+m (η)) K(η, α(η), . . . , αd+m+1 (η)) ≤ trdegK(α(η),...,αd+m (η)) K(α(η), . . . , αd+m+1 (η)) = trdegK(η,α(η),...,αd+m−1 (η)) K(η, α(η), . . . , αd+m (η)). By induction on m we obtain

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that trdegK(η,α(η),...,αd+m (η)) K(η, α(η), . . . , αd+m+1 (η)) ≤ δR − 1 for every m ∈ N hence trdegK K(η, α(η), . . . , αq (η)) ≤ trdegK K(η, α(η), . . . , αd (η)) + q(δR − 1). (5.2.5) Considering equalities (5.2.4) and (5.2.5) for large q we obtain that p = σtrdegK Kη ≤ δR − 1 < δR. In order to prove the last statement of the theorem, consider a realization η of the kernel R and a principal realization ξ of R such that η σ-specializes to ξ over K. Since the defining difference ideal of η is contained in the defining difference ideal of ξ, σ-trdegK Kη ≥ σ-trdegK Kξ = δR. The last inequality, together with part (v) of our theorem, implies that η is a principal realization of the kernel R and σ-trdegK Kη = σ-trdegK Kξ. Let a subindexing b0 of a0 be a special set, and let ζ and λ be subindexing of η and ξ, respectively, that correspond to the subindexing b0 . Part (ii) of our theorem shows that ζ and λ are σ-transcendence bases of Kη and Kξ, respectively, over K and σ-trdegK ζ Kη = σ-trdegK λ Kξ. It follows that the specialization of η to ξ is generic.  Let K be an inversive ordinary difference field with a basic set σ = {α} and let η = (η1 , . . . , ηs ) be an s-tuple with coordinates in some σ-overfield of K. Denoting the extension of α to Kη by the same letter α, we see that for any r ∈ N, η is a regular realization of the kernel R = (K(η, α(η), . . . , αr (η)), τ ) where τ is the restriction of α on K(η, α(η), . . . , αr−1 (η)). If r is sufficiently large, then η is the only principal realization of R. To prove this, let us consider the ring of σ-polynomials K{y1 , . . . , ys } in σindeterminates y1 , . . . , ys over K. Let P be the prime σ ∗ -ideal of K{y1 , . . . , ys } with generic zero η and let Σ = {g1 , . . . , gd } be a basis of P (that is, Σ is a set of generators of P as a perfect σ-ideal in K{y1 , . . . , ys }; by Theorem 2.5.11, one can always choose a finite basis of P ). Furthermore, let r denote the maximal number i such that some term of the form αi yj (1 ≤ j ≤ s) appears in one of the σ-polynomials g1 , . . . , gd . It is easy to see that if ζ = (ζ1 , . . . .ζs ) is a realization of the kernel R = (K(η, α(η), . . . , αr (η)), then ζ annuls every gj (1 ≤ j ≤ d) and therefore every σ-polynomial in P . It follows that η σ-specializes to ζ over K. If ζ is a principal realization of R, then Theorem 5.2.10(vi) shows that η is also a principal realization of this kernel and that η is the only (up to a σ-K-isomorphism) principal realization of R. We have arrived at the following statement. Proposition 5.2.11 Let K be an inversive ordinary difference field with a basic set σ and let η = (η1 , . . . , ηs ) be an s-tuple with coordinates in some σ-overfield of K. Then η is the unique principal realization of a kernel R over K such that every realization of R is a specialization of η over K.  Remark 5.2.12 Note that every kernel is equivalent to a kernel of length 0 or 1 in the sense that its realizations generate the same extensions. Indeed, with the

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notation of Definition 5.2.1 (with a kernel R = (K(a0 , . . . , ar ), τ ), r > 1), one can define two rs-tuples b0 and b1 with coordinates of the s-tuples a0 , . . . , ar−1 and a1 , . . . , ar , respectively. Then (K(b0 , b1 ), τ ) is a kernel of length 1 equivalent to R.

5.3

Difference Specializations

Recall that by a difference (σ-) specialization of an indexing a = {a(i) |i ∈ I} over a difference field K with a basic set σ we mean a σ-K-homomorphisms φ of K{a} into a σ-overfield of K (the indexing φa = {φa(i) |i ∈ I} is also called a σ-specialization of a over K). Obviously, an indexing b = {b(i) |i ∈ I} of elements of some σ-overfield of K is a σ-specialization of a if and only if {(τ (bi ) | i ∈ I, τ ∈ Tσ } is a specialization of the indexing {(τ (ai ) | i ∈ I, τ ∈ Tσ } in the sense of Definition 1.6.55. Also, every σ-specialization over K naturally defines a σ-specialization of every indexing in K{a} (but not necessarily of every indexing in Ka). If no confusion can result, we shall omit reference to a σ-field K and/or basic set σ while talking about difference specializations (so “specializations” will mean σ-specialization over a difference field under consideration). Also, we shall often use notation a for a specialization of an indexing a or its coordinates (so that a specialization of an indexing a = {a(i) |i ∈ I} will be denoted by a = {a(i) |i ∈ I}). If b = {a(j) |j ∈ J} is a subindexing of a (J ⊆ I) and a is a specialization of a, then b will denote the corresponding specialization of b. In particular, if the index set I is finite, I = {1, . . . , s}, b = (a(i1 ) , . . . , a(ik ) ) is a subindexing of a = (a(1) , . . . , a(s) ) (1 ≤ i1 < · · · < ik ≤ s) and a = (a(1) , . . . , a(s) ) is a specialization of a, then b will denote the specialization (a(i1 ) , . . . , a(ik ) ) of b. Recall that a specialization φ is called generic if it is a σ-isomorphism. Otherwise it is called proper. Note that if φ : a → b is a generic specialization over K, then φ has a unique extension to a difference isomorphism of Ka onto Ka. Proposition 5.3.1 Let K be a difference field with a basic set σ and let a = (a(1) , . . . , a(s) ) be a σ-specialization of an s-tuple a = (a(1) , . . . , a(s) ) over K. Furthermore, let b = (a(i1 ) , . . . , a(ik ) ) be a subindexing of a and b the corresponding σ-specialization of b. Then, if b is a σ-algebraically independent set over K, so is b, and hence σ-trdegK Ka(1) , . . . , a(s)  ≤ σ-trdegK a(1) , . . . , a(s) . PROOF. For every r ∈ N, our σ-specialization of b naturally induces a specialization of the indexing Σr = {τ (a(ij ) | (τ, j) ∈ Tσ (r) × {1, . . . , k}} (we shall denote this specialization by Σr ). By Theorem 1.6.58, trdegK K(Σr ) ≤ trdegK K(Σr ) for all r ∈ N, hence the leading coefficient of the difference dimension polynomial of the σ-field extension Kb/K associated with the set of σ-generators a(i1 ) , . . . , a(ik ) is less than or equal to the leading coefficient of the difference dimension polynomial of the σ-field extension Kb/K associated with the set of σ-generators a(i1 ) , . . . , a(ik ) . Applying Theorem 4.2.1(iii) we obtain the inequality σ-trdegK Kb ≤ σ-trdegK Kb which implies the first

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statement of the theorem and (after setting b = a) the desired inequality σ trdegK Ka(1) , . . . , a(s)  ≤ σ-trdegK Ka(1) , . . . , a(s) . Theorem 5.3.2 (R. Cohn) Let K be an ordinary difference field with a basic set σ = {α} and let η and ζ be two finite indexings in a difference overfield of K such that the field extension Kη, ζ/Kη is primary. Furthermore, let c be a nonzero element in K{η, ζ} and let B be a σ-transcendence basis of ζ over Kη. Then almost every σ-specialization η of η over K can be extended to a σ-specialization η, ζ of η, ζ over K such that (i) c = 0. (ii) Every σ-transcendence basis of ζ over Kη specializes to a σ-transcendence basis of ζ over Kη. (iii) If B denotes the σ-specialization of B, then trdeg(K η,B)∗ (Kη, ζ)∗ = trdeg(K η,B)∗ (Kη, ζ)∗ . PROOF. In order to satisfy (i), we extend ζ to the indexing ζ∪{c−1 } (considering c−1 as the last element of the extended indexing). Then, if a specialization η of η is extended to ζ ∪ {c−1 }, the specialization of c is not zero (otherwise, we would arrive at a contradiction 1 = cc−1 = 0). Also, if conditions (ii) and (iii) are valid for the indexing η and ζ ∪ {c−1 } (with the last indexing instead of ζ), these conditions are also valid for η and ζ. (For condition (ii) this statement is obvious. Furthermore, since c ∈ K{η, ζ}, B is a σ-transcendence basis of ζ ∪ c−1 over Kη, so if (iii) holds for η, ζ ∪ c−1 , it also holds for η, ζ.) Let η = (η (1) , . . . , η (s) ), ζ = (ζ (1) , . . . , ζ (m) ), and let K{y, z} denote the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys , z1 , . . . , zm over K (y denotes the s-tuple (y1 , . . . , ys ) and z denotes the m-tuple (z1 , . . . , zm )). Let P be the reflexive prime σ-ideal of K{y, z} with generic zero (η, ζ). Let t be a positive integer satisfying the following condition: there is a basis of the σ-ideal P which contains no σ-polynomial f with ordzj f > t for j = 1, . . . , m. (Since P has finite bases, such an integer t exists.) Let L denote the inversive closure of Kη. Then σ-trdegL Lζ = σ-trdegK η Kη, ζ (one can easily check this inequality taking into account that L = (Kη)∗ ). Let R denote the difference kernel (L(ζ, α(ζ), . . . , αt (ζ)), α). By Proposition 2.1.9(iii) and Theorem 1.6.48(i), the field extension L(ζ, α(ζ), . . . , αt (ζ) /L is primary. Applying Proposition 1.6.64 we obtain that there is an element c = 0 in (K{η})∗ with the following property: if φ is a specialization of the coordinates of αi (η) (i ∈ N) over K ∗ and φ(c) = 0, then φ has an extension to a nondegenerate specialization of ζ, α(ζ), . . . , αt (ζ) and a unique nondegenerate extension to a specialization of ζ, α(ζ), . . . , αt−1 (ζ). Let u = αj (c) be some transform of c (j ∈ N). Let η → η be a σ-specialization of η over K such that u = 0. This specialization extends to a unique specialization of the inverse transforms α−k (η) (k > 0) such that u = 0. Let ζ, . . . , αt (ζ) be a nondegenerate extension of this specialization (in the sense of Definition 1.6.62) to ζ, α(ζ), . . . , αt (ζ). Then ζ, α(ζ), . . . , αt−1 (ζ) and α(ζ), . . . , αt (ζ) are nondegenerate extensions of the specialization of the αi (η) to ζ, α(ζ), . . . , αt−1 (ζ) and α(ζ), . . . , αt (ζ), respectively.

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Since u = 0 and α(u) = 0, these nondegenerate extensions of the specialization of the αi (η) are unique (see Proposition 1.6.64). To complete the proof of the theorem we need the following lemma. Lemma 5.3.3 With the above notation, let L denote the inversive closure of Kη (the translation of this field will be denoted by the same letter α as the translation of K). Then α can be extended from L to an isomorphism τ of L(ζ, α(ζ), . . . , αt−1 (ζ)) onto L(α(ζ), . . . , αt (ζ)) such that τ (αi (ζ)) = αi+1 (ζ) (0 ≤ i ≤ t − 1). PROOF. Let β denote the translations of Lζ (which is an extension of α), let α be an extension of α to an isomorphism of L(ζ, α(ζ), . . . , αt−1 (ζ)), and let φ denote our specialization (indicated also as a bar over the corresponding letter). Then the mapping γ = α φβ −1 coincides with φ on (K{η})∗ and provides a nondegenerate extension of φ to a specialization of α(ζ), . . . , αt (ζ). By Proposition 1.6.64, such an extension is unique, so there is an L-isomorphism θ of L(γα(ζ), . . . , γαt (ζ)) onto L(α(ζ), . . . , αt (ζ)) such that θγαi (ζ) = αi (ζ), 1 ≤ i ≤ t. Then τ = θα is an isomorphism which is defined on φβ −1 ( (K{η})∗ [α(ζ), . . . , αt (ζ)]) = (K{η})∗ [ζ, α(ζ), . . . , αt−1 (ζ)], coincides with α on (K{η})∗ {η}, and maps αi (ζ) onto the field αi+1 (ζ) (0 ≤ i ≤ t − 1). Extending τ to an isomorphism of the quotient field L(ζ, α(ζ), . . . , αt−1 (ζ))  onto L(α(ζ), . . . , αt (ζ)) we obtain the desired extension of α. COMPLETION OF THE PROOF OF THE THEOREM Using the above notation and the extension τ , whose existence is established by the last lemma, we can consider a kernel R = (L(ζ, α(ζ), . . . , αt (ζ)), τ ). Denoting a principal realization of this kernel by ζ (this should not lead to any confusion), we obtain that η, ζ is a specialization of η, ζ (since η, ζ is a solution of a basis of P ). Thus, almost every specialization of η has an extension. Let λ be a σ-transcendence basis of ζ = (ζ (1) , . . . , ζ (m) ) over Kη and for any r ∈ N, let λr denote the set {αr (ζ (k) ) | ζ (k) ∈ λ}. Then for all sufficiently large r ∈ N, say for all r ≥ r0 (r0 is a fixed positive integer) and for every i = 1, . . . , m, the elements αr (ζ (i) ), . . . , αr (ζ (i) ) are algebraically dependent over L(λ, α(λ), . . . , αr (λ)). Let us choose t in the above part of the proof sufficiently large, so that for any σ-transcendence basis λ of ζ over Kη (there are only finitely many such bases) and for every i = 1, . . . , m, the elements αt (ζ (i) ), . . . , αt (ζ (i) ) are algebraically dependent over L(λ, α(λ), . . . , αt (λ)). Since the extension of the specialization of αi (η) is nondegenerate, (ζ (i) , α(ζ (i) ), . . . , αt (ζ (i) )) is algebraically dependent over L(λ, α(λ), . . . , αt (λ)) for i = 1, . . . , m (αi (λ) denotes the specialization of αi (λ)). We obtain that each αi (λ) is σ-algebraic over Lλ, hence λ contains a σ-transcendence basis of ζ over L. Since ζ is a principal realization, a special set ω for the kernel R is a σ-transcendence basis of ζ. Because of the nondegeneracy of the extension, ω is a special set of R and hence a σ-transcendence basis of ζ over L. It follows that if λ is a σ-transcendence basis of ζ over Kη, then λ is

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a σ-transcendence basis of ζ over Kη, that is, condition (ii) of the theorem is satisfied. In order to satisfy (iii) suppose, first, that B = ∅. Then the nondegeneracy implies that ord R = ord R, hence trdegK ∗ (Kη, ζ)∗ = trdegL Lζ = trdegL Lζ = trdeg(K η)∗ (Kη, ζ)∗ , so (iii) is satisfied. Now let B = ∅. By Theorem 4.4.1. one can choose a finite set S such that Kη, B, S is the algebraic closure of Kη, B in Kη, ζ. As we just proved, there exists a nonzero element a ∈ K{η, B, S} such that every specialization η, B, S of η, B, S over K with a = 0 can be extended to a specialization ζ of ζ with trdeg(K η,B,S)∗ (Kη, ζ)∗ = trdeg(K η,B,S)∗ (Kη, ζ)∗ = trdeg(K η,B)∗ (Kη, ζ)∗ . Since the field extension Kη, B, S/Kη is primary (see Theorem 1.6.48), the preceding part of the proof shows that (I) Almost every specialization η of η over K has an extension to a specialization η, B, S of η, B, S with B σ-algebraically independent over K and a = 0. Now, notice that any element u ∈ S is algebraic over Kη, B, hence it is a zero of some polynomial fu in one variable with coefficients in K{η, B}. If c is a nonzero coefficient of fu , then c can be written as a polynomial gc in elements τ (b) (τ ∈ Tσ , b ∈ B) with coefficients in K{η}. Let d be such a coefficient of gc , d = 0. If η is a specialization of η such that d = 0, a = 0, and B is σalgebraically independent over K (so that the specialization satisfies (I)), then c = 0. It follows that (II) If B and S are as in (I), then every element of S is algebraic over Kη, B. Let η be a specialization of η with the property described in (I) and let B and S be as in (I) (so that (II) also holds). In this case, as we have seen, ζ exists such that η, ζ is a specialization of η, ζ and trdeg(K η,B,S)∗ (Kη, ζ)∗ = trdeg(K η,B)∗ (Kη, ζ)∗ . This equality, together with (II), implies that trdeg(K η,B,S)∗ (Kη, ζ)∗ = trdeg(K η,B)∗ (Kη, ζ)∗ , so that our specialization satisfies condition (iii). This completes the proof of the theorem.  The following example is due to R. Cohn ([41, Chapter 7, Example 3]). It shows that one cannot drop the requirement that the difference field extension Kη, ζ/Kη in the hypothesis of Theorem 5.3.2 is primary. (Without this assumption one cannot even conclude that extensions of almost every specialization of η exist.) Example 5.3.4 Let us consider Q as an ordinary difference field whose basic set σ consists of the identity translation α. Let a be an element transcendental over Q and let Q(a) be treated as a σ-overfield of Q such that α(a) = −a. Furthermore, let b = a2 . Then α(b) = b, and b is transcendental over Q. If η ∈ Q, then η is a specialization of b over Q, since it lies in the irreducible variety of the σ-polynomial αy − y ∈ Q{y} and b is a generic zero of this variety. We are going to show that it is not true that almost every specialization of b can be extended to a specialization of b, a.

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Let η ∈ Q, η = 0. Then the specialization of b to η 2 cannot be extended to b, a. Indeed, if such an extension exists, then a would specialize to η or −η. but neither of these is a solution of the σ-polynomial αy + y annulled by a. Suppose that there is c ∈ Q{b}, c = 0, such that every specialization of b which does not specialize c to 0 can be extended to a. Then c ∈ Q[b], hence at most finite number of specializations of b specialize c to 0. At the same time, we just described an infinite set of specializations of b which cannot be extended to b, a.

5.4

Babbitt’s Decomposition. Criterion of Compatibility

Throughout this section K will denote an ordinary difference field with a basic set σ = {α}. An element a in some σ-overfield of K is said to be normal over K if K(a) is a normal field extension of K in the usual sense (see Section 1.6, in particular, Theorem 1.6.8). It is easy to check that if a is normal over K, then the field extension Ka/K is normal. Indeed, in this case K(a) is a splitting field over K of the minimal polynomial f (X) = Irr(a, K). Therefore, Ka is a splitting field over K of the family of polynomials (f (i) )i∈N where f (i) is a polynomial in K[X] obtained by applying αi to every coefficient of f . Let L be an algebraic σ-field extension of K. By Corollary 5.1.16, there exists an algebraic closure L of L which has a structure of a σ-overfield of L. Applying Theorem 1.6.13(v) we obtain that L contains a normal closure N of the field L over K. Let us show that N is a σ-overfield of L. Indeed, since N is a splitting field the family of polynomials {Irr(a, K) | a ∈ L} over K (see Theorem 1.6.13(i)), every element u ∈ N can be written f (a1 , . . . , am ) where f and g are polynomials in m variin the form u = g(a1 , . . . , am ) ables with coefficients in K, and a1 , . . . , am are roots of some polynomials Irr(b1 , K), . . . , Irr(bm , K), respectively, with b1 , . . . , bm ∈ L. Then α(u) = f1 (α(a1 ), . . . , α(am )) where the polynomials f1 and g1 are obtained by applyg1 (α(a1 ), . . . , α(am )) ing α to every coefficient of f and g, respectively, and α(a1 ), . . . , α(am ) are roots of the polynomials Irr(α(b1 ), K), . . . , Irr(α(bm ), K), respectively. Therefore, α(u) ∈ N , so that N is a σ-field extension of L. Proposition 5.4.1 Let K be an ordinary difference field with a basic set σ = {α}, L an algebraic σ-field extension of K, and N a normal closure of L over K (treated as a σ-field extension of L). Let a be an element of L such that ld(Ka/K) = 1 and let b be an element of N conjugate to a (that is, b is a root of Irr(a, K)). Then ld(Kb/K) = 1. PROOF. Let pi (x) denote the polynomial obtained by applying αi to every coefficient of Irr(a, K) (i = 0, 1, . . . ). Since Ka = K(a, α(a), α2 (a), . . . ), N contains a normal closure Na of Ka over K which is a splitting field of the

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family of polynomials {pi (x) | i = 0, 1, . . . } over K (see Theorem 1.6.13(iii)). By Proposition 4.3.12(ii), if ld(Ka/K) = 1, then Ka : K < ∞. It follows that Na : K < ∞ (see Theorem 1.6.13(iv)). Since Irr(b, K) = Irr(a, K), Na is also a normal closure of Kb over K. Applying Theorem 1.6.13(iv) once again we obtain that Kb : K < ∞ whence ld(Kb/K) = 1.  Corollary 5.4.2 Let K be an ordinary difference field with a basic set σ = {α} and let N be a σ-overfield of K such that the field extension N/K is normal.  Then the core NK is a normal field extension of K. Exercise 5.4.3 Let K be an inversive ordinary difference field and let L be a difference overfield of K such that the field extension L/K is algebraic. Prove that if N is a normal closure of L over K, then the core NK contains a normal closure of LK over K. The following example shows that, with the notation of the last exercise, NK needs not to coincide with the normal closure of LK over K. Example 5.4.4 Let K = Q(X) be the field of rational fractions in one indeterminate X over Q. Then K can be treated as an inversive ordinary difference field whose basic set σ consists of a translation α such that α(f (X)) = f (X + 1) for any f (X)√∈ K. Let L be a field extension of K obtained by adjoining to K elements 4 X + i (i ∈ N). (Formally speaking, one should adjoin to K a zero of the polynomial Y 4 − X ∈ K[Y ] and obtain a field K1 . Then we adjoin to K1 a zero of the polynomial Y 4 − (X + 1) ∈ K1 [Y ] and obtain a field K2 , then K2 is extended to a field K3 by adjoining a root of the polynomial Y 4 − (X + 2) ∈ K2 [Y ], etc. Finally, we define L as√the union of √ all fields Ki .) Treating L as a σ-field extension of K with α( 4 X + j)= 4 X + j √ + 1 (j ∈ ∞ n 4 = Kα ( X) = N) and applying Theorem 4.3.20 we obtain that L K n=0 √ √ ∞ 4 4 X + n, X + n + 1, . . . ) = K, so the normal closure of LK Q(X, n=0 over K coincides with K. On the other hand, the normal closure of L √ √ √ over K is the field N = Q(i, X, 4 X, 4 X + 1, . . . ) (i = −1). This field is a σ-overfield of K where the extension of α from L to N is defined by the condition α(i) √ = i. Using √ Theorem 4.3.20 once again we obtain that ∞ NK = n=0 Q(i, X, 4 X + n, 4 X + n + 1, . . . ) = Q(i, X)  K. Definition 5.4.5 Let L be a σ-overfield of K such that L = Ka for some element a ∈ L and the field extension L/K is algebraic. If K(a, α(a)) : K(a) = ld(L/K), then a is said to be a standard generator of L over K. If a is a standard generator of L over K such that K(a) : K is as small as possible, a is called a minimal standard generator of L over K. If L is a normal field extension of K, L = Ku for some u ∈ L, and K(u, α(u)) : K(u) = ld(L/K), then u is said to be a normal standard generator of L over K. If an element u ∈ L is a normal standard generator of L over K such that K(u) : K is as small as possible, u is said to be a minimal normal standard generator of L over K.

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In what follows we assume that L is a finitely generated σ-field extension of K such that the extension L/K is separably algebraic. Then one can always find a standard generator of L over K as follows. Suppose that L = KS for a finite set S ⊆ L and ld(L/K) = K(S, α(S), . . . , αi+1 (S)) : K(S, α(S), . . . , αi (S)) for some i ∈ N. By Theorem 1.6.17, one
can
choose
c ∈ L such that c is a linear combination of elements of S α(S) · · · αi (S) with coefficients in K and K(c) = K(S, α(S), . . . , αi (S)). Then L = Kc and K(c, α(c)) = K(S, α(S), . . . , αi+1 (S)), so that c is a standard generator of L over K. If, in addition, L/K is normal, then the set S  of all roots of all polynomials Irr(a, K), a ∈ S, is a finite set of σ-generators of L over K such that the field extension K(S  )/K is normal. As before, if ld(L/K) = K(S  , α(S  ), . . . , αj+1 (S  )) : K(S  , α(S  ), . . . , αj (S  )) for some j ∈ N, we can find an element u ∈ L such that K(u) = K(S  , α(S  ), . . . , αj (S  )). Then u is normal over K, L = Ku and ld(L/K) = K(u, α(u)) : K(u), so that u is a normal standard generator of L over K. In what follows, if L is a difference field extension of an ordinary difference field K, then LK denotes the core of this extension (see Definition 4.3.17). Lemma 5.4.6 Let K be an inversive ordinary difference field with a basic set σ = {α}. Let L be a finitely generated σ-field extension of K such that the field extension L/K is separably algebraic and normal. Then LK is a simple normal inversive σ-field extension of K such that ld(LK /K) = 1. Furthermore, (LK )K = LK . PROOF. The fact that LK is an inversive σ-field extension of K with ld(LK /K) = 1 was proved in Section 4.3 (see the remark after Definition 4.3.17). The normality of the extension LK /K follows from Corollary 5.4.2. Furthermore, Theorem 4.4.1 implies that LK is a finitely generated σ-field extension of K. Applying Proposition 4.3.12 we obtain that LK : K < ∞ hence LK = Ku for some element u ∈ LK . Without loss of generality we can assume that u is a standard generator of LK over K, that is K(u, α(u)) : K(u) = ld(LK /K) = 1. Then K(α(u)) ⊆ K(u). Since K is inversive, we obtain that K(u, α(u)) : K = K(u) : K = K(α(u)) : K = K(α(u)) : K hence K(α(u)) = K(u) and LK = K(u).  Finally, since ld(Ku/K) = 1, u ∈ (LK )K hence (LK )K = LK . Definition 5.4.7 A finitely generated ordinary difference (σ-) field extension L of a σ-field K is called benign if (i) L/K is an algebraic, normal and separable field extension. (ii) There exists an element u ∈ L such that L = Ku, u is normal over K and K(u) : K = ld(L/K). It follows from Theorem 4.3.22 that if L is a benign extension of an ordinary difference field K of zero characteristic, then LK = K. One of the main results of this section, Theorem 5.4.13, shows the existence of decomposition of a finitely generated ordinary difference field extension L/K, where L is separably algebraic and normal over K, into a finite sequence of benign extensions. We will follow [4] in the proof of this theorem.

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Definition 5.4.8 Let K be an ordinary difference field with a basic set σ = {α} and let L be a simple σ-field extension of K (that is, the extension L/K can be generated by a single σ-generator). Let u ∈ L be a σ-generator of L over K such that K(u) : K is minimal. Then u is called a minimal generator of L over K, and K(u) : K is said to be the minimal degree of L over K, written md(L/K). If md(L/K) = ld(L/K), then L is called a mild difference (σ-) field extension of K. Definition 5.4.9 Two difference fields K and L with the same basic set σ are called equivalent, written K  L, if there exist identical inversive closures of K and L. Let F ∗ be an inversive difference field with a basic set σ and let F1 and F2 be σ-subfields of F ∗ with the inversive closure F ∗ . Let K/F1 and L/F2 be σ-field extensions and let K ∗ and L∗ denote the inversive closures of K and L, respectively. Then we say that the σ-field extensions K/F1 and L/F2 are equivalent if there is a σ-F ∗ -isomorphism of K ∗ onto L∗ . (In most cases of such an equivalence we shall have K ∗ = L∗ .) Lemma 5.4.10 Let K and L be ordinary difference fields with basic set σ = {α} such that K  L. Let M denote the common inversive closure of K and L, and let u1 , . . . , un be elements in some σ-overfield of M such that for every i = 0, 1, . . . , n − 1, Ku1 , . . . , ui+1  is a mild σ-filed extension of Ku1 , . . . , ui  with minimal generator ui+1 . Then there exist m1 , . . . , mn ∈ N such that Lαr1 (u1 ), . . . , αri+1 (ui+1 ) is a mild σ-filed extension of Lαr1 (u1 ), . . . , αri (ui ) for all integers ri ≥ mi (0 ≤ i ≤ n − 1). PROOF. We proceed by induction on n. Since Ku1 /K is a mild difference field extension and u1 is a minimal generator of Ku1  over K, we obtain that K(u1 ) : K = K(u1 , α(u1 ), . . . , αi+1 (u1 )) : K(u1 , α(u1 ), . . . , αi (u1 )) for i = 1, . . . , n − 1. Therefore, if f (X) is the minimal polynomial of u1 over K, f (X) = Irr(u1 , K), then the polynomial fi+1 (X), obtained by applying αi+1 to every coefficient of f (X) (i = 0, . . . , n−1), is irreducible over K(u1 , α(u1 ), . . . , αi (u1 )). Since the coefficients of f (X) lie in K ⊆ M , fm1 (X) ∈ L[X] for sufficiently large m1 ∈ N. Then for every r1 ∈ N, r1 ≥ m1 , the polynomial fr1 +i (X) is irreducible over L(αr1 (u1 ), . . . , αr1 +i−1 (u1 )) (i = 0, 1, . . . ). Indeed, if it is not so, then for all sufficiently large j ∈ N, fr1 +i+j (X) would be reducible over K(αr1 +j (u1 ), . . . , αr1 +j+i−1 (u1 )) and hence over K(u1 , α(u1 ), . . . , αr1 +j+i−1 (u1 )) which is impossible. This completes the proof of the case n = 1. Suppose that the statement of the lemma is true for the case n − 1. Since Ku1 , . . . , un−1   L(αr1 (u1 ), . . . , αrn−1 (un−1 )) for any r1 , . . . , rn−1 ∈ N, the statement for u1 , . . . , un follows from the first part of the proof (the case n = 1) with un playing role of u1 , the field Ku1 , . . . , un−1  playing role of K, and  L(αr1 (u1 ), . . . , αrn−1 (un−1 )) playing role of L. Lemma 5.4.11 Let K and L be ordinary difference fields with basic set σ = {α} such that K  L. Let u1 , . . . , un be elements in some σ-field extension of the common inversive closure of K and L such that for every i=0, 1, . . . , n−1,

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the element ui+1 is normal over the field Ku1 , . . . , ui . Then there exist m1 , . . . , mn ∈ N such that αri +1 (ui+1 ) is normal over Lαr1 (u1 ), . . . , αri (ui ) for all ri ∈ N, ri ≥ mi (0 ≤ i ≤ n − 1). PROOF. Let f (X) = Irr(u1 , K). Since u1 is normal over K, the zeros of f (X) can be written as polynomials in u1 with coefficients in K. Therefore, there exists m1 ∈ N such that for any r1 ∈ N, r1 ≥ m1 , we have fr1 (X) ∈ L[X] and every zero of fr1 (X) can be written as a polynomial in αr1 (u1 ) with coefficients in L. (As before, for any d ∈ N, fd (X) denotes the polynomial obtained by applying αd to every coefficient of f (X).) Hence αr1 (u1 ) is normal over L. Now the statement of the lemma follows by induction on n.  The last two lemmas immediately imply the following result. Theorem 5.4.12 Let K and L be ordinary difference fields with basic set σ = {α}, let K  L, and let u1 , . . . , un be elements in some σ-field extension of the common inversive closure of K and L such that for every i = 0, 1, . . . , n − 1, Ku1 , . . . , ui+1  is a benign extension of Ku1 , . . . , ui  with minimal generator ui+1 . Then there exist m1 , . . . , mn ∈ N such that for all ri ∈ N, ri ≥ mi (0 ≤ i ≤ n − 1), Lαr1 u1 , . . . , αri+1 (ui+1 ) is a benign extension of  Lαr1 (u1 ), . . . , αri (ui ) with normal minimal generator αri+1 (ui+1 ). Theorem 5.4.13 (Babbitt’s Decomposition Theorem). Let K be an ordinary inversive difference field with a basic set σ = {α} and let L be a finitely generated σ-field extension of K such that the field extension L/K is separably algebraic and normal. Then there exist elements u1 , . . . , un ∈ L such that L  LK u1 , . . . , un  and for every i = 0, . . . , n − 1, LK u1 , . . . , ui+1  is a benign extension of LK u1 , . . . , ui  with normal minimal generator ui+1 . PROOF. Without loss of generality we can assume that LK = K. Let us set n = ld(L/K) and consider the following two possibilities: 1) L does not contain a proper normal field extension of K whose limit degree over K is less than n; 2) L contains such an extension of K. Case 1. Suppose that there is no intermediate difference field F between K and L such that F = K, F/K is normal and ld(F/K) < n. Let η be a normal standard generator of L over K which minimizes K(η) : K. Since K is inversive, K(α(η)) : K = K(η)  : K. Furthermore, it follows from Theorem 1.6.15(x) that K(α(η)) : K(η) K(α(η)) = K(η, α(η)) : K(η) = n. Let ζ be  a primitive element of the extension K(η) K(α(η))/K, so that K(ζ) = K(η) K(α(η)). Then α(ζ) ∈ K(α(η)) and K(ζ, α(ζ)) ⊆ K(α(η)). Since the field extension Kζ/K is normal, it follows from our assumption that either Kζ = K or ld(Kζ/K) = n. If Kζ = K, then K(α(η)) : K = K(η) : K = n, so that L is a benign extension of K with normal minimal generator  η. If ld(Kζ/K) = n, then n ≤ [K(ζ, α(ζ)) : K(ζ)] ≤ [K(α(η)) : K(η) K(α(η))] = n, so that K(ζ, α(ζ)) = K(α(η)) and Kζ = Kα(η). In this case ζ is a normal standard generator of Kα(η) over K and K(ζ) : K = [K(α(η)) : K] [K(η) : K] [K(ζ, α(ζ)) : K] = = . [K(ζ, α(ζ)) : K(ζ)] n n

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Since L = Kη does not have a normal standard generator over K of degree less than K(η) : K, the same can be said about the isomorphic field Kα(η), so that n = 1. Thus, ld(Kη/K) = 1 hence η ∈ LK = K and L = K. Case 2. Suppose that there is a normal σ-field extension F of K such that K
F ⊆ L and the limit degree k = ld(F/K) satisfies the condition 1 < k < n. As before we also suppose that LK = K. Furthermore, proceeding by induction on n, we assume that the theorem holds for extensions of limit degree less than n. Let L∗ and F ∗ denote inversive closures of L and F , respectively (F ∗ ⊆ L∗ ). Since the field extension F/K is normal, so is F ∗ /K. Therefore, the compositum LF ∗ is a normal finitely generated σ-field extension of F ∗ . By Lemma 5.4.6, the σ-field (LF ∗ )F ∗ is inversive and (LF ∗ )F ∗ = F ∗ w for some element w ∈ LF ∗ . Since the fields F ∗ w and L∗ are inversive, any transform of w generates the σ-field extension F ∗ w/F and without loss of generality we can assume that w ∈ L and w is a standard generator of F w over F . m  ci wi for some Then α(w) ∈ F (w), so that α(w) can be written as α(w) = i=0

c0 , . . . , cm ∈ F . Clearly, the restrictions of all K-automorphisms of L to the field F (w) form the set of all relative isomorphisms of F (w) over K. Therefore, m  ρ(ci )ρ(w)i where ρ is if α(v) is a conjugate of α(w) over K, then α(v) = i=0

the restriction of some K-automorphism of L to F (w). Since the field extension F/K is normal, ρ(ci ) ∈ F for i = 0, . . . , m. Therefore, every conjugate of α(w) over K lies in the field F (W ) where W denotes the set of all conjugates of w over K. It follows that F (W ) = F W  whence ld(F W /F ) = 1. Therefore, ld(F W /K) = [ld(F W /F )][ld(F/K)] = ld(F W /F ) = k. Since the field extension F/K is normal, so is F (W )/K. We obtain that F W  is a normal σ-field extension of K, F W K = K, and ld(F W /K) = k < n. By the induction hypothesis, there exist elements u1 , . . . , ul ∈ F W  such that F W   F u1 , . . . , ul  and for every i = 1, . . . , l, F u1 , . . . , ui  is a benign extension of F u1 , . . . , ui−1  with normal minimal generator ui . It follows from our previous considerations that LF ∗ is a finitely generated normal σ-field extension of F ∗ W . Since ld(F ∗ W /F ∗ ) = 1, we have F ∗ W  = (LF ∗ )F ∗ , so that the σ-field F ∗ W  is inversive. Furthermore, since ld(LF ∗ /F ∗ W ) ≤ ld(L/F W ) =

n ld(L/K) = < n, ld(F W /K) k

we can apply the induction hypothesis and obtain that there exist elements vl+1 , . . . , vs ∈ LF ∗ such that LF ∗  F ∗ W vl+1 , . . . , vs  and for every j = l + 1, . . . , s, F ∗ W vl+1 , . . . , vj  is a benign extension of F ∗ W vl+1 , . . . , vj−1  with normal minimal generator vj . Since F ∗ W   F W   Ku1 , . . . , ul , it follows from Theorem 5.4.12 that if we set uj = αm (vj ) (l + 1 ≤ j ≤ s) with m ∈ N sufficiently large, then Ku1 , . . . , ui+1  is a benign extension of Ku1 , . . . , ui  with normal minimal generator ui+1 (0 ≤ i ≤ s − 1). Obviously,

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CHAPTER 5. COMPATIBILITY, REPLICABILITY, MONADICITY L  LF ∗  F ∗ W vl+1 , . . . , vs   Ku1 , . . . , us .

This completes the proof of the theorem.



Let L be a finitely generated difference field extension of an inversive ordinary difference field K such that L/K is algebraic and normal. Then a finite sequence u1 , . . . , us of elements of L whose existence is established by Theorem 5.4.13 is said to define a benign decomposition of L over K. In what follows we are going to apply Babbitt Decomposition Theorem to the study of compatibility of ordinary difference field extensions. We begin with a statement which is a direct consequence of Proposition 2.1.7(v). As usual the inversive closure of a difference field F is denoted by F ∗ and the core of F over some its difference subfield K is denoted by FK . Furthermore, we assume that the field extensions considered below are separable. Proposition 5.4.14 Let K be an inversive ordinary difference field with a basic set σ = {α} and let L and M be σ-field extensions of K. Then (i) The σ-field extensions L/K and M/K are incompatible if and only if the extensions L∗ /K and M ∗ /K are incompatible. (ii) Suppose that the field extensions L/K and M/K are algebraic and equivalent (as difference field extensions of K). If F is another σ-overfield of K, then the σ-field extensions L/K and F/K are incompatible if and only if the extensions M/K and F/K are incompatible.  Our next result shows that the study of compatibility of difference field extensions can be reduced to the study of compatibility of finitely generated extensions. Theorem 5.4.15 Let K be an ordinary difference field with a basic set σ = {α} and let L and M be σ-overfields of K. Then the difference field extensions L/K and M/K are incompatible if and only if there exist intermediate σ-fields L and M  of L/K and M/K, respectively, such that the σ-field extensions L /K and M  /K are finitely generated and incompatible. PROOF. It follows from Theorem 5.1.6 that we may assume that the field extensions L/K and M/K are algebraic. Let S be a set of σ-generators of L over K and let B be a set of elements in some σ-overfield of M which is σ-algebraically independent over M and has the same cardinality as S. We suppose that a oneto-one correspondence between the sets S and B is fixed and consider K{B} as a ring of σ-polynomials in the set of σ-indeterminates B. Let P be the reflexive prime σ-ideal of K{B} consisting of σ-polynomials which are annulled when members of B are replaced by the corresponding members of S. Let Q be the perfect σ-ideal of M {B} generated by P . We are going to prove the following criterion: The σ-field extensions L/K and M/K are incompatible if and only if 1 ∈ Q.

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Indeed, if L/K and M/K are compatible, then there are elements in a σoverfield of M which when substituted for the corresponding elements of B annul every σ-polynomial of P and, therefore, every σ-polynomial of Q. Then 1 ∈ / Q. Conversely, suppose that 1 ∈ / Q. By Proposition 2.3.4 there exists a / Q . Then the reflexive prime σ-ideal Q of M {B} such that Q ⊆ Q and 1 ∈  difference factor ring M {B}/Q is an integral domain whose quotient σ-field N contains M . Let S  denote the image of the set B under the natural epimorphism M {B} → M {B}/Q . Since S  is a solution of the set of σ-polynomials of P , there is a σ-K-homomorphism of K{S} onto K{S  }. Since every element of S is algebraic over K (the extension L/K is algebraic), this mapping is actually a σ-isomorphism. Therefore, there is a σ-K-isomorphism of L into N whence the σ-field extensions L/K and M/K are compatible. Applying the criterion just proved, we obtain that P generates the unit ideal in M {B}. Then there exists a finite subset B1 of B such that P ∩ K{B1 } generates the unit ideal in M {B1 }. Let S1 be the finite subset of S that corresponds to B1 under the bijection between S and B, and let L = KS1 . Applying the above criterion to L (instead of L) and P ∩ K{B1 } (instead of P ) we obtain that the σ-field extensions L /K and M/K are incompatible. Application of the same procedure to M produces a finitely generated σ-field extension M  of K such that M  ⊆ M and the extensions L /K and M  /K are incompatible.  Proposition 5.4.16 Let F and N be difference overfields of an inversive ordinary difference field K with a basic set σ = {α}. Let G be a benign σ-field extension of F and let φ be a σ-K-isomorphism of F onto a σ-subfield F  of N . Then φ can be extended to a σ-K-isomorphism of G into N . In particular (if F = K), a benign extension G/K is compatible with every σ-field extension of K. PROOF. Let η be a minimal generator of G over F , let f (X) = Irr(η, F ), and let g(X) be a polynomial in F  [X] obtained from f (X) by replacing the coefficients of f by their images under φ. If ζ is any zero of g(X), then φ can be extended to a field isomorphism φ0 of F (η) onto F  (ζ) such that φ(η) = ζ. Let f1 (X) denote the polynomial obtained from f (X) by applying α to every coefficient of f . Since η is a minimal generator of a benign extension of F , the polynomial f1 (X) is irreducible over F (η). Let g1 (X) be a polynomial obtained from f1 (X) by replacing the coefficients of f by their images under φ0 . Since f1 (X) ∈ F [X] and φ is a σ-isomorphism, α(ζ) is a zero of g1 (X), so we can extend φ0 to an isomorphism φ1 of F (η, α(η)) onto F  (ζ, α(ζ)) such that φ1 (α(η)) = α(ζ). A straightforward induction argument now yields the statement of the proposition.  The following statement gives a criterion of the compatibility for separably algebraic normal difference field extensions. Proposition 5.4.17 Let K be an ordinary difference field with a basic set σ = {α} and let L and M be difference (σ-) field extensions of K such that the field extensions L/K and M/K are normal. Then a necessary and sufficient

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condition for the compatibility of the σ-field extensions L/K and M/K is the compatibility of the difference field extensions LK /K and MK /K. PROOF. Clearly, the incompatibility of LK /K and MK /K implies the the incompatibility of L/K and M/K. Therefore, in order to prove the theorem, one should show that if L/K and M/K are incompatible, so are LK /K and MK /K. Suppose that LK and MK have σ-K-isomorphisms into a σ-field extension H of K. Without loss of generality we can assume that the field H is algebraically closed (see Corollary 5.1.16). Furthermore, it follows from Theorem 5.4.15 and Theorem 5.1.6 that we can suppose that the σ-field extensions L/K and M/K are finitely generated and separably algebraic. Finally, we can assume that the σ-field K is inversive. (Indeed, if it is not so, we may replace K with its inversive closure K ∗ and replace L and M with K ∗ L and K ∗ M , respectively. Then the cores LK and MK will be replaced by their inversive closure (see Theorem 4.3.24). Clearly, these replacements do not affect compatibility.) Under all these assumptions, let u1 , . . . , um and v1 , . . . , vn define benign decompositions of the extensions L/K and M/K, respectively. By Proposition 5.4.14(ii), in order to complete the proof, it is sufficient to show that LK u1 , . . . , um  and MK v1 , . . . , vn  have σ-K-isomorphisms into H. This fact, however, is a direct consequence of Proposition 5.4.16, so our proposition is proved.  The following considerations are aimed at removing the condition of normality in Proposition 5.4.17 and obtaining a criterion of compatibility of arbitrary difference field extensions. Note that the proof of the corresponding statement in [41, Chapter 7] assumed that the normal closure of the core of a difference field extension L/K coincides with the core of the extension N/K where N is the normal closure of L over K. Unfortunately, this is not always so, see Example 5.4.4. In his recent paper on compatibility theorem (to appear in the Pacific Journal of Mathematics) R. Cohn has presented a correct proof of the general form of this theorem, which can be considered as a criterion of compatibility. Our proof is a slight modification of R. Cohn’s arguments. In what follows we treat some fields as ordinary difference fields with respect to different translations. In order to avoid confuses and make our arguments in such considerations clear, it is sometimes convenient to use different notation for a difference field and its underlying field, that is, the same field treated as a regular (non-difference) field. We adopt the following notation and terminology. ˆ will denote If K is an ordinary difference field with a basic set σ = {α}, then K the underlying field of K (i. e., if a capital letter denotes some difference field, then the same letter with a “hat” denotes the corresponding underlying field). ˆ α). Furthermore, if an overfield L of K The field K will be also denoted by (K, can be treated as a difference overfield of K with respect to several extensions of α to endomorphisms of L, β is one of such extensions, and S is a set of difference generators of L/K with respect to β, we write L = KSβ .

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Lemma 5.4.18 Let K be an ordinary difference field with a basic set σ = {α}, ˆ be a normal field extension of the field K ˆ and let β and γ be two extensions let N ˆ , β) ˆ . Let Nβ and Nγ denote the difference fields (N of α to endomorphisms of N ˆ and (N , γ), respectively, treated as difference overfields of K. Furthermore, let Fβ and Fγ denote the cores of Nβ and Nγ over K, respectively. Then Fˆβ = Fˆγ . PROOF. Obviously, it is sufficient to show that Fˆβ ⊆ Fˆγ (the opposite β is a finite exinclusion would follow by symmetry). Let a ∈ Fˆβ . Then Ka tension of K (see Proposition 4.3.12). Let p(X) = Irr(a, K) and let pi (X) be the polynomial obtained by applying αi to every coefficient of p(X) (i ∈ N). ˆ /K is normal, N ˆ contains the set A of all roots of all Since the field extension N β contained in N ˆ ˆ. is the normal closure of Ka polynomials pi (X) and K(A)  ˆ ˆ ˆ Since Kaβ : K < ∞, K(A) : K < ∞ (see Theorem 1.6.13(iv)). Furthermore, since a is a root of p(X), γ i (a) is a root of pi (X) for every i ∈ N. It follows that γ over K ˆ contained in N ˆ . Therefore, ˆ K(A) is also the normal closure of Ka γ : K ˆ < ∞ hence a ∈ Fˆγ . Thus, Fˆβ ⊆ Fˆγ . Ka  Lemma 5.4.19 If L is a difference overfield of an ordinary difference field K, then the core of L over LK coincides with LK . PROOF. Let a be an element of the core of L over LK . Then ld(LK a/LK ) = 1. Therefore, ld(Ka/K) ≤ ld(LK a/K) = ld(LK a/LK )ld(LK /K) = 1. It  follows that ld(Ka/K) = 1, hence a ∈ LK . Lemma 5.4.20 Let K be an ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K. Let L∗ be the inversive closure of L, K ∗ the . a σ-overfield of K ∗ L contained inversive closure of K contained in L∗ , and L ∗ ∗ ∗ . in L . Then LK ∗ = (LK ) where (LK ) denotes the inversive closure of LK . . K ∗ . Then a ∈ L∗ and a is separably algebraic over K ∗ , PROOF. Let a ∈ L hence there exists r ∈ N such that αr (a) ∈ L and αr (a) is separably algebraic over K. Applying Corollary 4.3.6 we obtain that ld(Kαr (a)/K) = ld((Kαr (a))∗ /K ∗ ) = ld((Ka)∗ /K ∗ ) = ld(K ∗ a/K ∗ ) = 1. Therefore, αr (a) ∈ LK whence a ∈ (LK )∗ . On the other hand, Theorem 4.3.19 . K ∗ is inversive. Furthermore, LK ⊆ L . K ∗ . Indeed, if shows that the σ-field L a ∈ LK , then 1 ≤ ld(K ∗ a/K ∗ ) ≤ ld((Ka)∗ /K ∗ ) = ld(Ka/K) = 1, . K ∗ hence (LK )∗ = L .K ∗ . K ∗ . It follows that (LK )∗ ⊆ L hence a ∈ L



Lemma 5.4.21 The following statements are equivalent. (i) Let K be an ordinary difference field and let L and M be two algebraic difference field extensions of K such that the extensions LK /K and MK /K are compatible. Then the extensions L/K and M/K are also compatible.

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(ii) If L is an algebraic difference field extension of an ordinary difference field K such that LK = K, then L/K is compatible with every difference field extension of K. (iii) If L is an algebraic difference field extension of an ordinary difference field K such that LK = K, then L/K is compatible with every normal difference field extension of K. PROOF. Clearly, (i) implies (ii), and (ii) implies (iii). Also, it is easy to see that (iii) implies (ii). Indeed, let L/K be an algebraic ordinary difference field extension such that LK = K and let M be any difference overfield of K. Then the normal closure N of the field M over K can be equipped with a structure of a difference overfield of M (see the remark at the beginning of this section). Applying statement (iii) we obtain that difference field extensions L/K and N/K are compatible. Therefore, L/K and M/K are compatible as well. It remains to prove statement (i) assuming that statement (ii) is true. Let L/K and M/K be algebraic ordinary difference field extensions such that LK /K and MK /K are compatible. Then there are difference K-isomorphisms of LK and MK , respectively, into a difference overfield F of K. Without loss of generality we can assume that LK and MK are contained in F . Since the core of L over LK is LK (see Lemma 5.4.19), statement (ii) implies that the difference field extensions L/LK and F/LK are compatible, so there are difference LK - (and therefore K-) isomorphisms of L and F , respectively, into a difference overfield G of K. Without loss of generality we can assume that G contains F as its difference subfield. Since the core of M over MK is MK , one can apply statement (ii) and obtain that the extensions M/MK and G/MK are compatible, so there are difference MK - (and therefore K-) isomorphisms of G and M into a difference overfield H of K. It follows that there are difference K-isomorphisms of L and M into H, so L/K and M/K are compatible.  Theorem 5.4.22 (Criterion of Compatibility). Let K be an ordinary difference field with a basic set σ = {α} and let L and M be difference (σ-) field extensions of K. Then a necessary and sufficient condition for the compatibility of the σ-field extensions L/K and M/K is the compatibility of the difference field extensions LK /K and MK /K. PROOF. Because of Theorems 5.1.6 and 5.4.15 it is sufficient to prove the theorem under the assumption that the difference field extensions L/K and M/K are finitely generated and separably algebraic. Furthermore, without loss of generality we may assume that the difference field K is inversive. Indeed, let . = K ∗ L and M / = K ∗ M . K ∗ denote the inversive closure of K and let L Suppose that the σ-field extensions LK /K and MK /K are compatible, that is, there are σ-K-isomorphisms of LK and MK into some σ-overfield Ω of K. Then these isomorphisms can be naturally extended to σ-K ∗ -isomorphisms of the inversive closures (LK )∗ and (MK )∗ of LK and MK , respectively, into the . K ∗ = (LK )∗ and M /K ∗ = (MK )∗ , inversive closure Ω∗ of Ω. By Lemma 5.4.20, L

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. K ∗ → Ω∗ and M /K ∗ → Ω∗ . If our theorem hence there are σ-K ∗ -isomorphisms L ∗ . ∗ and M //K ∗ are comis true for K (instead of K), then the extensions L/K patible, hence L/K and M/K are compatible. Finally, Lemma 5.4.21 shows that it is sufficient to prove the following result: (∗) Let K be an inversive ordinary difference field with a basic set σ = {α}, let L and M be finitely generated separably algebraic difference field extensions of K, and let LK = K. Then the difference field extensions L/K and M/K are compatible. In order to prove this statement, let us denote the basic translation of L by αL (this is an extension of α to an endomorphism of L) and consider a ˆ of L ˆ over K ˆ as an underlying field of a difference overfield N normal closure N of L with a translation αN (the existence of an extension of αL to an endomorˆ is established at the beginning of this section). Thus,we obtain a phism of N ˆ , αN ). ˆ α) ⊆ L = (L, ˆ αL ) ⊆ N = (N chain of difference field extensions K = (K, By Proposition 4.3.12, the core F = NK is a finite extension of K, so F = K(a) for some element a ∈ F . Let p(y) be the minimum polynomial of a ˆ which will be also treated as a difference polynomial in the ring K{y} of over K difference polynomials in one difference indeterminate y over K. Furthermore, ˆ obtained by applying for every i ∈ Z, let pi (y) denote the polynomial in K[y] i α to every coefficient of p(y). Since K ⊆ M , p(y) ∈ M {y}. By Theorem 7.2.1, whose proof is independent of the results of this section and their consequences, there is a solution η of the difference polynomial p(y) in some difference field ˆ αΩ ) of M . Without loss of generality we can assume that extension Ω = (Ω, ˆ ˆ is normal (otherwise one can replace Ω by a difference field the extension Ω/K ˆ over K). ˆ whose underlying field is the normal closure of Ω Let S = {p(y), p1 (y), p−1 (y), . . . } ⊆ K{y}. Since the difference field exteni (a) is a root of pi (y) contained in F , sion F/K is normal and for every i ∈ Z, αN ˆ ˆ F is a splitting field of the family S over K. On the other hand, for every i ∈ Z, i ˆ Since the extension Ω/ ˆ K ˆ is normal, Ω ˆ (η) is a root of pi (y) contained in Ω. αΩ ˆ of the family S over K. ˆ Furthermore, since for every contains a splitting field G zero b of a polynomial pi (y), αΩ (b) is a zero of pi+1 (y), the restriction αG of ˆ is an isomorphism of the field G ˆ onto itself. Thus, G = (G, ˆ αG ) is a αΩ to G ˆ αΩ ). difference subfield of Ω = (Ω, ˆ It follows from Theorem 1.6.5(iii) that there is a K-isomorphism µ of Fˆ ˆ (both fields are splitting fields of the same set of polynomials over K). ˆ onto G ˆ coincides Then β = µ−1 αG µ is an endomorphism of Fˆ whose restriction on K with α. Thus, Fβ = (Fˆ , β) is a difference overfield of K. The following diagram illustrates the arrangement of the fields under consideration.

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ˆ N

   M η



@ @



@

ˆ L

ˆ Ω

@

Fˆ

µ

 - G ˆ @ @

ˆ M @

ˆ K

ˆ K

ˆ β(Fˆ )/K is also such Since Fˆ is a finite normal separable extension of K,  ˆ ˆ ˆ =K ˆ = α(K) ˆ ˆ ˆ ˆ an extension. Furthermore,  F L = LK =K and β(F ) αL (L) ˆ ˆ ˆ ˆ ˆ ˆ ˆ (since K = α(K) ⊆ β(F ) αL (L) ⊆ F L = K). It follows that the field ˆ and L/ ˆ K ˆ are linearly disjoint (see the last part of Theorem extensions Fˆ /K 1.6.24). Applying Theorems 1.6.43 and 1.6.5(ii) to the diagram α ¯

- N ˆ :  1     @   @ αL   @    @  β ˆ L Fˆ

ˆ N

ˆ K

α

- α(K) ˆ =K ˆ

ˆ into itself (it is shown by we obtain that there exists an isomorphism α ¯ from N ˆ coincide with β and a dotted line) such that the restrictions of α ¯ on Fˆ and L ˆ Fˆ into N ˆ which αL , respectively. (Theorem 1.6.43 gives an isomorphism φ of L extends αL and β, and Theorem 1.6.5 (ii) (see also Theorem 1.6.8(v)) implies ˆ →N ˆ , since the existence of an extension of φ to the desired isomorphism α ¯:N ˆ is a splitting field of the family {Irr(a, K) | a ∈ L} ˆ over L ˆ Fˆ .) N ˆ , α). ¯ Then N  is a normal difference extension of K with the core Let N  = (N Fβ = (Fˆ , β) (see Lemma 5.4.18). Since µ is a difference K-isomorphism of Fβ onto G ⊆ Ω, the difference field extensions Fβ /K and ΩK /K are compatible. (As usual, ΩK denotes the core of the difference field Ω over K.) Taking into account that both N  /K and Ω/K are normal, one can apply Proposition 5.4.17 and ˆ αL ) obtain that these difference field extensions are compatible. Since L = (L, and M are intermediate difference fields of the extensions N  /K and Ω/K, respectively, the extensions L/K and M/K are compatible as well.  Note that Theorem 5.4.22 can be proved without referring to the existence theorem for ordinary difference polynomials (Theorem 7.2.1), if one uses the algebraic closure of M as the difference field Ω in our proof of Theorem 5.4.22. Indeed, by Corollary 5.1.16, Ω can be treated as a difference overfield of M

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whose translation αΩ is an extension of the translation αM of M (which, in ˆ turn, extends the translation α of K). Clearly, Ω contains a splitting field G of the set of polynomials S = {p(y), p1 (y), p−1 (y), . . . } ⊆ K{y} (we use the notation of the proof of Theorem 5.4.22) and αΩ (G) ⊆ G. Then we can consider ˆ and complete the prove as above. the isomorphism µ of the field Fˆ onto G Theorems 5.4.15 and 5.4.22 imply the following statement. Corollary 5.4.23 Let K be an ordinary difference field with a basic set σ and let L and M be two σ-field extensions of K. Then the following statements are equivalent. (i) L/K and M/K are incompatible. (ii) There exist finitely generated σ-field extensions L and M  of K such that L ⊆ L, M  ⊆ M , and L /K and M  /K are incompatible. (iii) LK /K and MK /K are incompatible. (iv) LK /K and M/K are incompatible.  Corollary 5.4.24 Let K be an ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K such that the field extension L/K is primary. Furthermore, let M and N be two σ-overfields of L such that ML /L is equivalent to LMK /L, and NL /L is equivalent to LNK /L. Then M/L and N/L are compatible if and only if M/K and N/K are compatible. PROOF. Let L be a σ-overfield of L such that there is a σ-K-isomorphism φ of MK into L and L = Lφ(MK ). Since the field extension φ(MK )/K is algebraic, L is the free join of φ(MK ) and L. By Proposition 1.6.49(i), φ(MK )/K and L/K are quasi-linearly disjoint. Let L be another σ-overfield of L such that there is a σ-K-isomorphism ψ of MK into L and L = Lφ(MK ). By Theorem 1.6.43(ii), there is an isomorphism ρ of L onto L which is identical on L and coincides with φψ −1 on ψ(MK ). Obviously, ρ is a σ-L-isomorphism of L onto L , so that the described extension L of L is unique up to a σ-L-isomorphism. Of course, the similar statement holds for the corresponding construction for NK . Since the compatibility of M/L and N/L trivially implies the compatibility of M/K and N/K, we assume that M/K and N/K are compatible and show that the same is true for M/L and N/L. First, we observe that in our case the extensions MK /K and NK /K are also compatible. Let F be a σ-overfield of K such that there are σ-K-isomorphisms µ and ν of MK and NK , respectively, into F . Without loss of generality, we can also assume that F is the compositum of µ(MK ) and ν(NK ). Then F/K is an algebraic extension and the σ-field extensions F/K and L/K are compatible by Theorem 5.1.6. let G be a σoverfield of L such that F has a σ-K-isomorphism into G. Then MK and NK have σ-K-isomorphisms into G. Let M  and N  denote the images of MK and NK , respectively, under these isomorphisms. By the first part of the proof, LM   is σ-L-isomorphic to LMK  and LN   is σ-L-isomorphic to LNK , therefore the extensions LMK /L and LNK /L are compatible. It follows that

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the equivalent extensions ML /L and NL /L are compatible. Applying Theorem 5.4.22 we obtain that the extensions M/L and N/L are also compatible.  Theorem 5.4.25 Let K be an ordinary difference field with a basic set σ = {α} and let L = KS where the set S is σ-algebraically independent over K. Furthermore, let M be a σ-overfield of L. Then the σ-field extensions ML /L and LMK /L are equivalent. PROOF. If a ∈ MK , then Ka is a finite field extension of K. Therefore, KS ∪{a} is a finite extension of KS (see Theorem 1.6.2) and ld(La/L) = 1. Furthermore, a is algebraic and separable over L (this element is algebraic and separable over K), so that a ∈ ML . Since L ⊆ ML , we obtain that LMK  ⊆ ML . Let b ∈ ML and let
K  denote the algebraic closure of the field K in its u ∈ K  , then overfield Lb = KS {b} which is a σ-subfield of ML . If
∞ KS ∪ {u} is a finite field extension of KS. Since the set i=0 αi (S) is algebraically independent over K  , Ku is a finite extension of K (see Theorem 1.6.28(v)). Furthermore, even if Char K > 0, the element u is separable over K. Indeed, by Theorem 1.6.28(vii), the fields Ku and L are linearly disjoint whence Irr(u, K) = Irr(u, L). Since u ∈ ML and therefore u is separable over L, we obtain that u is separable over K. Since b ∈ ML , there exists k ∈ N such that every αi (b) (i ≥ 0) lies in KS(b, α(b), . . . , αk (b)). Furthermore, there is a positive integer
r r such that the elements b, α(b), . . . , αk (b) are algebraic over the field K( i=0 αi (S)). Let k+h (b) is algebraic over h be
∞a positive integer such that k + h > r. Then α K( i=r+1 αi (S)).
r

∞ Let U = i=r+1 αi (S), V = i=0 αi (S), E = K  (U {αk+h (b)}), F =


K  (V
{b, α(b), . . . , αk (b)}), and
G = K  (U V {b, α(b), . . . , αk (b)}) = KS {b}. Since U , V and U V are transcendence bases of E, F and G, respectively, over the field K  , we obtain that G is the free join of E and F over K  (see Theorem 1.6.40(ii) and Theorem 1.6.37(v)). Since the field K  is algebraically closed in E, it follows from Proposition 1.6.49(i) that the field extensions E/K  and F/K  are quasi-linearly disjoint. In order to complete the proof, suppose first that Char K = 0. Then E and F are linearly disjoint over K  . Since αk+h (b) ∈ KS(b, α(b), . . . , αk (b)) ⊆ F (U ), f where f, g ∈ F [U ]. Let {wi | i ∈ J} be a basis of F as a vector αk+h (b) = g space over K  . Then   λij wi uj , g = µij wi uj (5.4.1) f= i,j

i,j

where uj are distinct products of nonnegative powers of elements of U , the elements λij and µij lie in K  , and each sum is finite. Since g = 0, there are nonzero coefficients among µij . Without loss of generality we can assume that by linear µ11 = 0. Since the set {wi | i ∈ J} is linearly independent  over E  disjointness, the equation αk+h (b)g = f implies αk+h (b) µ1j uj = λ1j uj j

j

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where the summation is extended over all indices j such that at least one of the elements λ1j , µ1j in the sums in (5.4.1) is different from zero. Since the power products uj are linearly independent over K  and µ11 = 0, we have µ1j uj = 0 whence αk+h (b) ∈ K  (U ) ⊆ K  S ⊆ LMK . Thus, if b is j

any element of ML , then some transform of b lies in LMK . Since LMK  ⊆ ML , the extensions ML /L and LMK /L are equivalent. . and F. of E Now suppose that Char K = p > 0. then the perfect closures E . and F , respectively, are linearly disjoint over the perfect closure K of K  . Setting f αk+h (b) = as before, and letting the wi now denote a basis of F. as a vector g . we obtain expressions similar to those in (5.4.1): f =  λij wi uj space over K, i,j  . and µ11 = 0. Then and g = µij wi uj where λij and µij lie in K i,j

αk+h (b)

 j

µ1j uj =



λ1j uj .

(5.4.2)

j

Since the power products uj are linearly independent over any algebraic exten. the sum in the left-hand side of (5.4.2) is sion of K  and, in particular, over K, k+h . not 0, hence α (b) ∈ K(U ). Then there exists a positive integer m such that m (αk+h (b))p ∈ LMK , so that the element αk+h (b) is purely inseparable over LMK . since this element is in the core ML , it is separable over L and therefore over LMK . By Theorem 1.6.19(i), αk+h (b) ∈ LMK . This completes the proof of the theorem.  Theorem 5.43.25 and Corollary 5.4.24 imply the following statement. Corollary 5.4.26 Let K be an ordinary difference field with a basic set σ = {α} and let L = KS where the set S is σ-algebraically independent over K. Let M be a σ-field extension of K such that the extensions L/K and M/K are equivalent. Furthermore, let F and G be two σ-field extensions of M such that F/K and G/K are separably algebraic and normal. Then F/M and G/M are compatible if and only if F/K and G/K are compatible.  We conclude this section with some applications of the preceding results to the study of difference specializations. The correspondent results are due to R. Cohn [38], [41, Chapter 7]. Theorem 5.4.27 Let K be an ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K such that the σ-field extension L/K is finitely generated and primary. Let M be a finitely generated σ-field extension of L such that the extensions ML /L and LMK /L are equivalent. Furthermore, let η and ζ be finite indexings such that L = Kη and M = Lζ. Finally, let B be a σtranscendence basis of ζ over L, and let c be a nonzero element of K{η, ζ}. Then almost every σ-specialization η¯ of η over K such that M/K and K¯ η /K are compatible can be extended to a σ-specialization η¯, ζ¯ of η, ζ with the properties (i), (ii), and (iii) of Theorem 5.3.2.

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PROOF. Let ζ ∗ denote a finite set of σ-generators of ML over L. Then, in order to prove the theorem, it is sufficient to show that almost every σspecialization η¯ of η over K such that Kη, ζ/K and K¯ η /K are compatible can be extended to a σ-specialization of η, ζ ∗ over K which does not annul a pre-assigned nonzero element u ∈ K{η, ζ ∗ }. Let ζ  be a finite set of σ-generators of MK over K. Since the extensions ML /L and LMK /L are equivalent, there is m ∈ N such that αm (u) ∈ Kη, ζ   and αm (a) ∈ Kη, ζ   for every element a in ζ ∗ . Since the elements of ζ  are algebraic over Kη, there exists an element v ∈ K{η} with the following properties: za (a) If a is an element of ζ ∗ , then αm (a) can be represented as a fraction v where za ∈ K{η, ζ  }. w for some w ∈ K{η, ζ  }. (b) αm (u) = v Since w is algebraic over Kη, there is a polynomial f (X) in one variable X with coefficients in Kη such that f (w) = 0. Let e denote the term of f (X) free of X. Since u = 0, we can assume that e = 0. Clearly, if a σ-specialization of η over K does not annul ve and extends to a σ-specialization of η, ζ  over K, then it also extends to a σ-specialization of η, ζ ∗ over K not annulling u. To complete the proof of the theorem, it is sufficient to show that every (and therefore almost every) σ-specialization η¯ of η over K such that Kη, ζ/K and K¯ η /K are compatible extends to a σ-specialization of η, ζ  over K. Assuming that the conditions stated in the last paragraph hold, we observe first that the compatibility of Kη, ζ/K and K¯ η /K imply the compatibility η /K. Let K¯ η , ζ¯ / be a σ-overfield of K¯ η  such that Kζ   of Kζ   and K¯   ¯ has a σ-K-isomorphism into K¯ η , ζ /, the image of ζ being ζ¯ . Let us show  ¯ η ), αi (ζ¯ ) (i ∈ N) that η¯, ζ is a σ-specialization of η, ζ  over K, that is, the αi (¯ i i  form a specialization of α (η), α (ζ ) over the field K. Indeed, it is clear that η ) form a specialization of the αi (η) and the αi (ζ¯ ) of the αi (ζ  ). Also, the αi (¯ the αi (ζ  ) are algebraic over K and the field extension K(η, α(η), α2 (η), . . . )/K is primary by our hypothesis. Applying Proposition 1.6.49(i) we obtain that the fields K(ζ  , α(ζ  ), α2 (ζ  ), . . . ) and K(η, α(η), α2 (η), . . . )/K are quasi-linearly η ) and αi (ζ¯ ) form a disjoint over K. Now Proposition 1.6.65 shows that the αi (¯  specialization of the αi (η) and αi (ζ  ). This completes the proof. Theorem 5.4.28 Let K be an ordinary difference field of zero characteristic with a basic set σ = {α}. Let L = Kη, ζ be a finitely generated σ-field extension of K where η = (η1 , . . . , ηk ) and ζ = (ζ1 , . . . , ζl ) are finite indexings with elements η1 , . . . , ηk σ-algebraically independent over K. Furthermore, let B be a σ-transcendence basis of ζ over Kη and let c be a nonzero element of K{η, ζ}. Finally, let K{y1 , . . . , yk } be the ring of σ-polynomials in σ-indeterminates y1 , . . . , yk over K. Then there exists a nonzero σ-polynomial A ∈ K{y1 , . . . , yk } such that if an η /K is compatible with indexing η¯ = (¯ η1 , . . . , η¯k ) is not a solution of A and K¯ ¯ of (η, ζ) with the properties (i), (ii), L/K, then there is a σ-specialization (¯ η , ζ) and (iii) of Theorem 5.3.2.

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PROOF. It follows from Theorems 5.4.25 and 5.4.27 that there exists a nonzero element µ ∈ K{η} such that every σ-specialization of η which does not specializes µ to zero and satisfies the stated property of compatibility can be extended to a σ-specialization of η, ζ with the properties (i), (ii), and (iii) of Theorem 5.3.2. Since the components of η are σ-algebraically independent over K, any indexing of k elements in a σ-overfield of K is a specialization of η. If µ is expressed as a σ-polynomial in η1 , . . . , ηk with coefficients in K, then one can choose the desired σ-polynomial A as the result of replacing each ηi in this expression of µ  by the corresponding σ-indeterminate yi (1 ≤ i ≤ k). The following example shows that the condition of σ-algebraic independence of η1 , . . . , ηk in the formulation of Theorem 5.4.28 is essential. Example 5.4.29 Let G and H denote ordinary difference fields constructed on Q(i) (i2 = −1) with the use of the identical automorphism and complex conjugation, respectively. (We consider Q as a difference field whose basic set σ consists of the identical translation α and denote the translations of G and H by the same symbol α.) To avoid confusion, we shall denote i, treated as an element of the field H, by j, so that G = Qi and H = Qj. Let us adjoin to H a σ-algebraically independent over H element η and let ζ = jη. If η¯, ζ¯ is a σ-specialization of η, ζ over Q with η¯ = 0, then H is σ-Q ¯ 2 ζ ζ¯ = −1 and the translation sends the element isomorphic to Q , since η¯ η¯ ζ¯ ζ¯ to − . η¯ η¯ It is easy to see that the variety of a linear difference polynomial αk y − iy (k = 1, 2, . . . ) of the difference polynomial ring G{y} is irreducible. Let λk be a generic zero of this variety. Then for every k = 1, 2, . . . , λk = 0, G ⊆ Qλk , and λk is a σ-specialization of η over Q, since η is σ-algebraically independent over Q. This σ-specialization cannot be extended to a σ-specialization λk , ζ¯ of η, ζ over Q. Indeed, if such an extension exists, then Qλk , ζ/K would contain the intermediate σ-subfield G and a σ-subfield σ-Q-isomorphic to H. This would contradict the incompatibility of G/Q and H/Q established in Example 5.1.2. The following theorem shows that “in most cases” an indexing and its specialization generate compatible difference field extensions. Theorem 5.4.30 Let K be an ordinary difference fields with a basic set σ = {α} and let η be a finite indexing whose coordinates lie in some σ-overfield of K. Then almost every σ-specialization of η over K generates a σ-field extension of K compatible with Kη/K. PROOF. Let S be the separable part of Kη over K. By Theorem 4.4.1, there exists a finite set ζ such that S = Kζ. Furthermore, there is a nonzero element µ ∈ K{η} such that every member of ζ is a quotient of some element

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of K{η} by µ. Then every σ-specialization η¯ of η over K which does not specialize µ to 0 can be extended to a σ-specialization η¯, ζ¯ of η, ζ over K. Then ¯ . . . is a specialization of ζ, α(ζ), α2 (ζ), . . . over the field K (in the ¯ α(ζ), ¯ α2 (ζ), ζ, sense of the Definition 1.6.55). Since all elements of all αi (ζ) (i ∈ N) are algebraic over K, they have only generic specializations. It follows that there is a ¯ . . . with αi (ζ) correspondK-isomorphism from K(ζ, α(ζ), α2 (ζ), . . . ) to K(ζ), i ¯ ¯ and since ing to α (ζ), i = 0, 1, . . . . Therefore, Kζ is σ-K-isomorphic to Kζ, ¯ ⊆ K¯ Kζ η , the σ-field extensions Kζ/K and K¯ η /K are compatible. Applying Theorem 5.1.6 we obtain that Kη/K and K¯ η /K are also compatible. This completes the proof.  Theorems 5.4.28 and 5.4.30 lead to a generalization if the difference version of the Nullstellensatz (Theorem 2.6.5). In what follows, K denotes an ordinary difference field with a basic set σ = {α} and K{y1 , . . . , ys } denotes the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K. As before, we say that an indexing (a set) is σ-algebraic over K if every its coordinate (respectively, every element of the set) is σ-algebraic over K. Lemma 5.4.31 With the above notation, let M and N be distinct varieties over K{y1 , . . . , ys }. Then their subsets consisting of solutions σ-algebraic over K are distinct. PROOF. Suppose, first, that M is a nonempty irreducible variety over K{y1 , . . . , ys } and N is a variety over K{y1 , . . . , ys }, which does not contain M. Then there exists a σ-polynomial A ∈ Φ(N ) \ Φ(M) (we use the notation of Section 2.6). Let (η1 , . . . , ηs ) be a generic zero of M and let K  be the algebraic closure of K in Kη1 , . . . , ηs . Let L be a completely aperiodic σ-overfield of K  , which is σ-algebraic over K  . (For example, L is the σ-field obtained by adjoining to K  a generic zero of the σ-polynomial α(z) − z − 1 in the ring of σ-polynomials in one σ-indeterminate z over K  .) Then Theorem 5.1.6 implies that the σ-field extensions L/K and Kη1 , . . . , ηs /K are compatible. Let ζ be a σ-transcendental basis of the set η = {η1 , . . . , ηs } over K and let λ = η \ ζ. Without loss of generality we can assume that the elements of ζ and λ form a k-tuple (η1 , . . . , ηk ) (1 ≤ k ≤ s) and the (s − k)-tuple (ηk+1 , . . . , ηs ) denoted by the same letters ζ and λ. Let u denotes the indexing (y1 , . . . , yk ) of our σ-indeterminates and let µ be the element of K{η1 , . . . , ηs } obtained by / Φ(M), µ = 0. substituting ηi for yi in A (1 ≤ i ≤ s). Since A ∈ By Theorem 5.4.28, there exists a nonzero σ-polynomial B ∈ K{u} with the following property: if ζ¯ is an indexing of k elements in a σ-overfield of K, ζ¯ is not ¯ a solution of B and Kζ/K is compatible with Kζ, λ/K, then there exists a ¯ ¯ ¯ is σ-algebraic over σ-specialization ζ, λ of ζ, λ such that every coordinate of λ ¯ ¯ K and the σ-specialization of µ is not 0. Then ζ, λ is an s-tuple in M which does not annul B and therefore this s-tuple does not lie in N . Since L/K and Kη1 , . . . , ηs /K are compatible, it follows from Proposition 4.5.3 that ζ¯ can be chosen as a set of elements of L. Then every coordinate of ¯ λ ¯ is σ-algebraic over K. We have ζ¯ is σ-algebraic over K hence the s-tuple ζ,

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obtained that M contains a solution, which is σ-algebraic over K and does not lie in N . this completes the proof.  Theorem 5.4.32 Let K be an ordinary difference field with a basic set σ and K{y1 , . . . , ys } an algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys over K. Let S ⊆ K{y1 , . . . , ys } and let A be a σ-polynomial in K{y1 , . . . , ys }. Then the following conditions are equivalent. (i) A is annulled by every solution of the set S which is σ-algebraic over K. (ii) A lies in the perfect ideal {S} of the ring K{y1 , . . . , ys }. PROOF. The implication (ii) =⇒ (i) is obvious, so we just need to show that (i) implies (ii). Suppose that (i) holds, but A ∈ / {S}. By Theorem 2.6.5, the variety M(S) strictly contains M(S ∪ {A}). Applying Lemma 5.4.31 we obtain that there is an s-tuple η ∈ M(S) \ M(S ∪ {A}), which is σ-algebraic over K. This contradicts condition (i).  We complete this section with a result on the existence of simple (that is, generated by one element) incompatible difference field extensions obtained with the use of the criterion of compatibility. Theorem 5.4.33 Let K be an inversive ordinary difference field with a basic set σ = {α}, let L = Kη1 , . . . , ηm  be a σ-field extension of K such that σtrdegK L = m (so that η1 , . . . , ηm is a σ-transcendence basis of L over K). Let F and G be finitely generated incompatible σ-field extensions of the inversive closure L∗ of L. Then there exist elements a ∈ F and b ∈ G such that the σfields Ka and Kb are algebraic extensions of K and the σ-field extensions Ka/K and Kb/K are incompatible. PROOF. Since the σ-field extensions F/L∗ and G/L∗ are incompatible, the same is true for the extensions N (F )/L∗ and N (G)/L∗ where N (F ) and N (G) denote the normal closures of F and G, respectively, over L∗ . Therefore, without loss of generality we can assume that F and G are normal extensions of L∗ . By Theorem 5.4.22, the σ-field extensions FL∗ /L∗ and GL∗ /L∗ are incompatible. It follows (see Proposition 2.1.7(v)) that there exist σ-field extensions F  and G of L such that F  ⊆ F , G ⊆ G, the extensions F  /L and G /L are algebraic and incompatible, and ld(F  /L) = ld(G /L) = 1. Then the σ-fields F  and G are generated by the adjunction to L some elements a and b, respectively, which are algebraic over K. If Ka/K and Kb/K are compatible, then one can find a σ-field extension H of K containing σ-subfields σ-isomorphic to Ka and Kb. Adjoining m elements annulling no nonzero σ-polynomial with coefficients in H (and hence annulling no nonzero σ-polynomial with coefficients in K), we obtain a σ-overfield of H which contains a σ-subfield σ-isomorphic to L. We obtain a σfield extension of L which contains σ-subfields σ-isomorphic to La and Lb. Thus, the extensions La/L and Lb/L are compatible, contrary to the fact  that F  /L and G /L are incompatible. This completes the proof.

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Replicability

Definition 5.5.1 Let K be a difference field with a basic set σ and L/K, M/K two σ-field extensions of K. Then the number of σ-K-isomorphisms of L into M is called the replicability of L/K in M/K. The replicability of a σ-field extension L/K is defined as the maximum of replicabilities of L/K in all σ-field extensions of K, if this maximum exists, or ∞ if it does not. It is easy to see that if a difference field extension L/K is finitely generated (in the difference sense), then it is sufficient to define the replicability of L/K considering only difference fields in the universal system over K (see Definition 2.6.1). Furthermore, if L/K is a difference field extension and K ∗ and L∗ are the inversive closures of K and L, respectively, then the replicability of L∗ /K ∗ is the same as the replicability of L/K. (We leave the proof of this fact to the reader as an exercise.) The following statement is a version of the R. Cohn’s “fundamental replicability theorem” (see [41, Chapter 7, Theorem II]) in the case of partial difference fields. Theorem 5.5.2 Let K be difference field with a basic set σ and L a σ-field extension of K. Then a necessary condition for finite replicability of L/K is that every element of L have a transform of some order algebraic over K. This condition is sufficient if L is finitely generated over K. PROOF. Suppose, first, that the σ-fields K and L are inversive and the replicability of L/K is finite. Let L be the algebraic closure of K in L, and suppose that L contains elements no transforms of which are algebraic over K. We shall show that for every i ∈ N, there exists an inversive σ-overfield Li of K and 2i σ-K-isomorphisms of L into Li with the property that if a ∈ L \ K  , then the images of a under these isomorphisms are all distinct. Clearly, these images are not in the σ-field Li of all elements of Li algebraic over K. We construct the sequence of σ-fields Li by induction starting with L0 = L. Suppose that the fields L0 , . . . , Li − 1 (i ≥ 1) has been found. By Corollary 5.1.5 (with Li−1 for K and Li−1 for both L1 and L2 in the conditions of the Corollary), we find a σ-overfield M of Li−1 such that Li−1 /Li−1 has two σ-Li−1 isomorphisms φ and ψ into M with φ(Li−1 ) and ψ(Li−1 ) quasi-linearly disjoint over Li−1 and M their compositum. Clearly, the σ-field M is inversive, and for any distinct elements a, b ∈ Li−1 \ Li−1 , the elements φ(a), φ(b), ψ(a), ψ(b) are all distinct (see Theorem 1.6.43). Thus, we can set Li = M . Since the replicability of L/K in Li /K is at least 2i , the existence of the sequence L0 , L1 , . . . implies that the replicability of L/K is ∞. This completes the proof of the necessity in the case of inversive difference field extensions. The necessity in the general case immediately follows from the observation made before the theorem: if K ∗ and L∗ are the inversive closures of K and L, respectively, then the replicability of L∗ /K ∗ is the same as the replicability of L/K. To complete the proof of the theorem, assume that L = KS, where S = {a1 , . . . , am } is a finite set, and that every element of L has a transform in L , the algebraic closure of K in L. Then there exist τ1 , . . . , τm ∈ Tσ such that

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τi (ai ) ∈ L (1 ≤ i ≤ m). Setting τ = τ1 . . . τm we obtain that τ (S) ⊆ L . Since every element of L has a transform in Kτ (S), L/K and Kτ (S)/K have equal replicabilities. Therefore, it remains to prove that the replicability of Kτ (S)/K is finite. Let N be any σ-overfield of K. Then distinct σ-Kisomorphisms of Kτ (S) into N contract to distinct field K-isomorphisms of K(τ (S)) in to N . Since the number of such K-isomorphisms is finite (it cannot exceed K(S) : K), this completes the proof of the theorem.  Corollary 5.5.3 Let K be a difference field with a basic set σ, K{y1 , . . . , ys } the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and P a reflexive prime σ-ideal in K{y1 , . . . , ys }. Furthermore, let η = (η1 , . . . , ηs ) be a generic zero of P and L = Kη1 , . . . , ηs . If trdegK ∗ L∗ > 0, then there exist σ-overfields of K containing arbitrarily many generic zeros of P . PROOF. It is easy to see that if P satisfies the stated condition, there exists an element of L no transform of which is algebraic over K. Applying Theorem 5.5.2 we obtain that L/K has finite replicability. Therefore, there are infinitely many distinct images of η under σ-K-isomorphisms into σ-overfields of K. Since each such an image is a generic zero of P , this completes the proof.  The following statement is a direct consequence of Theorems 5.1.10 and 5.4.22. Proposition 5.5.4 Let L be a difference field extension of an ordinary difference field K with a basic set σ such that LK = K. Then every σ-isomorphism of K into a σ-overfield M of L extends to a σ-isomorphism of L into a σ-overfield of M .  It follows from the last proposition that if an ordinary difference field extension L/K is separably algebraic and normal, then its replicability is not less than the replicability of LK /K. The following result strengthens this statement. Proposition 5.5.5 Let K be an ordinary difference field with a basic set σ, let L be σ-field extension of K, and let F be an intermediate σ-field of the extension L/K. Then the replicability of L/K is greater than or equal to the replicability of LF /K. PROOF. Let M be a σ-overfield of K such that there are k distinct σ-Kisomorphisms φ1 , . . . , φk of LF into M . We are going to show that there are k distinct σ-K-isomorphisms of L into some σ-overfield of K. Actually, we will construct inductively a sequence M1 ⊆ · · · ⊆ Mk of σ-overfields of M such that each φi extends to a σ-K-isomorphism of L into Mi . By Theorem 5.4.22 we may assume that L ⊆ M . Applying Proposition 5.5.4 we obtain that φ1 has an extension to a σ-K-isomorphism of L into some σ-overfield M1 of M . Suppose that σ-fields M1 , . . . , Mj−1 (2 ≤ j ≤ k) for which each φi (1 ≤ i ≤ j − 1) extends to a σ-K-isomorphism of L into Mi has been constructed. By Proposition 5.5.4, there exists an extension of φj to a σ-K-isomorphism of L into a σ-overfield Mj of Mj−1 . This completes the construction of the sequence, and therefore the proof of the proposition. 

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Definition 5.6.1 Let K be a difference field with a basic set σ and L a difference (σ-) overfield of K. The difference field extension L/K is called monadic if its replicability is 1. (In this case we also say that L is monadic over K.) If the replicability of L/K is greater than 1, the extension is said to be amonadic. Thus, a difference field extension L/K is monadic if L does not admit two distinct σ-K-isomorphisms into any σ-field extension of K. A monadic difference field extension L/K is called properly monadic if L = K and not every element of L has a transform in K. Example 5.6.2 Let L be the field of rational fractions in one indeterminate x over the field of complex numbers C. Let t = x3 and let K = C(t). Let us treat L as an ordinary difference field with a basic set σ = {α} such that α : f (x) → f (x2 ) for every f (x) ∈ L. Clearly, α is an isomorphism of L into itself and K is a σ-subfield of L. Let us show that the identical automorphism is the only σ-K-automorphism of L (despite the fact that there are non-identical K-automorphisms of L, since the field extension L/K is normal, see Theorem 1.6.9). Moreover, we shall show that if M is any σ-overfield of L, then there is at most one σ-K-isomorphism of L into M . Indeed, let φ and ψ be two such σ-K-isomorphisms. Let y = φ(x) and z = ψ(x). Then y 3 = z 3 = t, α(y) = y 2 , and α(z) = z 2 . Clearly, in order to prove that φ = ψ, it is sufficient to show that z = y. Since y 3 = z 3 , we obtain that z = ωy where ω 3 = 1. Then ω ∈ C ⊆ K and α(ω) = ω. Furthermore, since α(z) = z 2 , ωα(y) = ω 2 y 2 hence α(y) = ωy 2 . On the other hand, α(y) = y 2 = 0. It follows that ω = 1 and z = y. Thus, the σ-field extension L/K is monadic. Definition 5.6.3 A difference field extension L/K with a basic set σ is called pathological if either L is monadic over K or L/K is incompatible with some other σ-field extension of K. The first two statements of the next proposition are direct consequences of Proposition 2.1.7(v). The third statement follows from Proposition 5.5.5. Proposition 5.6.4 Let K be an inversive difference field with a basic set σ. Then (i) A σ-field extension L of K is monadic if and only if the inversive closure L∗ of L is a monadic extension of K. (ii) If L1 and L2 are equivalent algebraic finitely generated σ-field extensions of K, then the extension L1 /K is monadic if and only if L2 /K is monadic. (iii) If a σ-field extension L/K is monadic, then the extension LK /K is also monadic. 

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The following two theorems describe situations when difference field extensions are amonadic. Both statements are due to A. Babbitt [2]. Theorem 5.6.5 Let K be an inversive ordinary difference field with a basic set σ = {α} and let Kη be a benign σ-field extension of K with minimal normal standard generator η. Suppose that η ∈ / K, and let L be an intermediate σ-field of Kη/K. Then L is an amonadic extension of K. PROOF. Suppose that the extension L/K is monadic. Without loss of generality we can assume that L = Kζ for some element ζ ∈ L. let N denote the inversive closure of Kη and let M be the inversive closure of L in N . Furthermore, let G = Gal(N/K) be the Galois group of the extension N/K in the usual algebraic sense and let G(M ) be a subgroup of G corresponding to the subfield M (that is, G(M ) = M  if one uses the notation of Theorem 1.6.51). Since Kη/K is a benign extension with minimal normal standard generator η and K is inversive, for any two distinct integers i and j, the fields K(αi (η)) and K(αj (η)) are linearly disjoint and have isomorphic Galois groups over K. Therefore, we can represent G as an infinite direct product of finite groups all isomorphic to the Galois group Gη of K(η) over K. Using this observation we will denote elements g ∈ G as strings (. . . , g0 , g1 , g2 , . . . ) where gi (i ∈ Z) denote elements of Gη and their isomorphic images in the components of the mentioned decomposition of G. Furthermore, for any g = (. . . , g0 , g1 , g2 , . . . ) ∈ G we set g ∗ = α−1 gα. Notice that that if gα(x) = αg(x) for every element x ∈ M , then g ∈ G(M ) , since L is a monadic extension of K and by Proposition 5.6.4 the same is true of M . Also, it is easy to see that if g = (. . . , g0 , g1 , g2 , . . . ), then g ∗ = (. . . , g1 , g2 , g3 , . . . ), that is, if g0 acts on η, g1 on α(η), etc., then g1 acts on η, g2 on α(η), etc. in g ∗ . With this definition of g ∗ , it is clear that if M is a monadic extension of K and g −1 g ∗ ∈ G(M ) , then g ∈ G(M ) . Let ζ ∈ Kαm (η), . . . , αm+i (η) (m ∈ N). Then we can choose elements gm , . . . , gm+i , where gm+j acts on αm+j (η) (0 ≤ j ≤ i) so that (. . . , gm−1 , gm , g2 , . . . , gm+i , gm+i+1 , . . . ) ∈ G\G(M ) for any choice of . . . , gm−1 , gm−2 , . . . ; gm+i+1 , gm+i+2 , . . . . It follows that for some integers k (e.g., for k = i) it is possible to select elements gm , . . . , gm+k acting on αm (η), . . . , αm+k (η), respectively, such that g = (. . . , gm−1 , gm , . . . , gm+k , gm+k+1 , . . . ) ∈ G \ G(M ) for any choice of . . . , gm−1 , gm−2 , . . . ; gm+k+1 , gm+k+2 , . . . . Let such a k be chosen as small as possible. Let g = (. . . , gm−1 , gm , . . . , gm+k , gm+k+1 , . . . ) be an arbitrary completion of gm , . . . , gm+k to an element (a “string”) of G and let h = g −1 g ∗ . / G(M ) . Setting Since g ∈ / G(M ) and M is a monadic extension of K, h ∈ −1 hi = gi gi+1 (i ∈ Z) we can easily see that hm , . . . , hm+k−1 are determined by gm , . . . , gm+k , while the other hi being arbitrary (since any desired set hm−1 , hm−2 , . . . ; hm+k , hm+k+1 , . . . can be obtained by some choice of a completion of gm , . . . , gm+k to a string in G). If k > 0, this contradicts the choice of k. If k = 0, then all hi are arbitrary and can be chosen so that the corresponding h is the identity e of G. We obtain that e ∈ / G(M ) which is impossible. This completes the proof of the theorem. 

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Theorem 5.6.6 Let K be an inversive ordinary difference field with a basic set σ = {α} and let L = Kη be a σ-field extension of K generated by an element η ∈ L. Furthermore, suppose that the extension L/K is algebraic and ld(L/K) > 1. Then L is an amonadic extension of K. PROOF. Let N be the normal closure of L over K. Then N is a finitely generated σ-field extension of K (it immediately follows from Theorem 1.6.13(iii)). Since ld(Kη/K) > 1, η ∈ / NK . Let elements ξ1 , . . . , ξn define a benign decomposition of N over K. For every i = 1, . . . , n, let Mi denote the inversive closure of NK ξ1 , . . . , ξi  and let M0 = NK . Furthermore, let l denote the smallest positive integer for which η ∈ Ml (1 ≤ l ≤ n). Since NK ξ1 , . . . , ξl−1 ξl  is a benign extension of NK ξ1 , . . . , ξl−1  and the extensions NK ξ1 , . . . , ξl−1 /K and Ml−1 /K are equivalent, we obtain from Theorem 5.4.12 that Ml−1 αm (ξl ) is a benign extension of Ml−1 for some sufficiently large m ∈ N. Since for all sufficiently large r ∈ N we have αr (η) ∈ Ml−1 αm (ξl ) \ Ml−1 , it follows from Theorem 5.6.5 that Ml−1 αr (ξl ) is an amonadic extension of Ml−1 for such large r. Applying Proposition 5.6.4(ii) we obtain that Ml−1 η is an amonadic extension of Ml−1 . This implies that L = Kη is an amonadic extension of K.  The further study of monadic extensions of ordinary difference fields will need consideration of Galois groups of difference field extensions and the concept of coordinate extensions (see Definition 5.6.19) below). R. Cohn introduced this concept in [41] and used it in his investigation of monadicity. In what follows we present the results of this study. Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and L a σ ∗ -overfield of K. As usual, Gal(L/K) denotes the corresponding Galois group, that is, the group of all automorphisms (not necessarily σ-automorphisms) that leave the field K fixed. It is easy to see that the mappings α¯i : θ → αi−1 θαi (θ ∈ Gal(L/K), 1 ≤ i ≤ n) are automorphisms of the group Gal(L/K); they are called the induced automorphisms of Gal(L/K). Definition 5.6.7 With the above notation, a subgroup B of Gal(L/K) is called σ-stable if α ¯ i (B) = B for i = 1, . . . , n. If α¯i (b) = b for every b ∈ B, αi ∈ σ, the subgroup B is called σ-invariant. The largest σ-invariant subgroup of Gal(L/K) consists of all σ-automorphisms of L that leave the field K fixed. This group is called the difference (or σ-) Galois group of L/K; it is denoted by Galσ (L/K). Definition 5.6.8 A difference field extension L/K with a basic set σ is called σ-stable in a σ-overfield M of L if every σ-K-automorphism of M maps L into itself. The extension L/K is said to be strongly σ-stable in M if every σ-Kisomorphism of L into M maps L onto itself. A difference (σ-) field extension L/K is called σ-stable (strongly σ-stable) if it is σ-stable (respectively, strongly σ-stable) in every σ-overfield of L. Exercise 5.6.9 Show that a difference (σ-) field extension L/K is σ-stable if and only if for any σ-overfield M of L, every σ-K-automorphism of M maps L onto itself.

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Exercise 5.6.10 Prove that an ordinary algebraic difference field extension L/K is σ-stable if and only if it is normal. The proof of the following statement is an easy exercise that we leave to the reader. Proposition 5.6.11 Let K be an inversive difference field with a basic set σ and let M be a σ ∗ -overfield of K. Then (i) If L is an intermediate σ-field of M/K, then the Gal(M/L) is σ-stable subgroup of Gal(M/K). (ii) If H is a σ-stable subgroup of Gal(M/K) and F = {a ∈ M | φ(a) = a for every φ ∈ H}, then F is a σ ∗ -overfield of K and H is a subgroup of Gal(M/F ). ∗ (iii) For any  intermediate σ -field L of the extension M/K, one has  Galσ (M/K) Gal(M/L) = Galσ (M/L). Exercise 5.6.12 Let K be a difference (not necessarily inversive) difference field with a basic set σ, L a σ-overfield of K, and H = {g ∈ Gal(L/K) | αg = gα for every α ∈ σ}. Show that a) If A is a subgroup of H, then L(A) = {x ∈ L | g(x) = x for every g ∈ A} is a difference (σ-) field. b) If the σ-field L is inversive, then L(A) is inversive and H = Gal(L/K ∗ ) where K ∗ denotes the inversive closure of K in L. Example 5.6.13 As in Example 5.1.11, let Q be treated as an inversive ordinary difference field with the identity translation α, and let a and i denote the positive fourth root of 2 and the square root of −1, respectively. Let L = Q(i, a) be considered as a σ ∗ -overfield of Q such that α(i) = −i and α(a) = −a. Then K = Q(i) is an intermediate σ ∗ -field of L/Q and Gal(L/K) is the cyclic group of order 4 with generator β such that β(a) = ia. In this ¯ (β) = β 3 . case, α ¯ (β)(a) = α−1 βα(a) = α−1 β(−a) = α−1 (−ia) = −ia, hence α It follows that Galσ (L/Q) consists of the identity automorphism and β 2 . The fixed field M (Galσ (L/Q)) of this group is Qa2 , which is the only intermediate σ ∗ -field of L/Q (except of Q and L). In what we consider ordinary difference fields. In this case we shall use the notation introduced in section 5.4: if K is a difference field with a basic set ˆ will denote the underlying field of K (that is, if a capital letter σ, then K denotes some difference field, then the same letter with a “hat” denotes the ˆ σ) or corresponding underlying field). The field K will be also denoted by (K, ˆ (K, α) if K is an ordinary difference field with a basic set σ = {α}. (However, if no confusion can arise, we still can talk about algebraic, separable, normal, etc. σ-field extensions of K meaning that the underlying fields of such extensions ˆ are algebraic, separable, normal, etc. field extensions of K.) Proposition 5.6.14 Let K be a difference field with a basic set σ and let L be a σ-overfield of K such that the extension L/K is σ-stable. Then the inversive closure of L is algebraic over the inversive closure of K.

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PROOF. Without loss of generality we may assume that the σ-field K is ¯ denote the algebraic closure of inversive and L is a σ ∗ -overfield of K. Let K ¯ = K ¯ for every α ∈ σ). K in L treated as a σ ∗ -overfield of K (clearly, α(K) ¯ = L, it follows from Corollary 5.1.5 (with L2 = L1 ) that there exists a If K ¯ σ-overfield M of L and a σ-K-isomorphism φ : L → M such that L and φ(L) ¯ and M = Lφ(L). By Theorem 1.6.43, there are quasi-linearly disjoint over K, ˆ which contracts to φ on L and to φ−1 on φ(L). It is an automorphism ψ of M follows that ψ is a σ-K-automorphism of M which does not map L into itself. This contradiction with the σ-stability of L/K completes the proof.  Definition 5.6.15 An automorphism φ of a group G with identity e is called regular if φ(g) = g for every g ∈ G, g = e. Proposition 5.6.16 Let L be an inversive difference (σ ∗ -) field extension of an inversive ordinary difference field K with a basic set σ = {α}. Then (i) If the extension L/K is σ-stable and the induced automorphism α ¯ of the group Gal(L/K) is regular, then L/K is monadic. (ii) The extension L/K is monadic if and only if it is strongly σ-stable and the automorphism α ¯ is regular. (iii) If the extension L/K is algebraic and normal, then L/K is monadic if and only if this extension is σ-stable and α ¯ is regular. PROOF. The first two statements are direct consequences of the definitions of monadic, σ-stable, and strongly σ-stable extensions. Statement (iii) follows from the obvious fact that if L/K is algebraic, normal, and σ-stable, then it is strongly σ-stable as well.  Proposition 5.6.17 Let K be a difference field with a basic set σ, L a σoverfield of K, and M a σ-overfield of L. If the σ-field extension L/K is σ-stable in M then Galσ (M/L) is a normal subgroup of Galσ (M/K) and the factor group Galσ (M/K)/Galσ (M/L) is isomorphic to the group of all σ-K-automorphisms of L which are contractions of σ-K-automorphisms of M . PROOF. Let a ∈ L, φ ∈ Galσ (M/K), and λ ∈ Galσ (M/L). Since the extension L/K is σ-stable in M , φ(a) ∈ L, hence λφ(a) = φ(a). Then φ−1 λφ(a) = a, so that φ−1 λφ ∈ Galσ (M/L). Clearly, the elements of Galσ (M/K) contract to σ-K-automorphisms of L, and two elements of Galσ (M/K) contract to the same element of Galσ (L/K) if and only if thee lie in the same coset of Galσ (M/L) in Galσ (M/K). We obtain a one-to-one correspondence between elements of Galσ (M/K)/Galσ (M/L) and the elements of Galσ (L/K) which are contractions of σ-K-automorphisms of M to L. It is easy to see that this correspondence is a group isomorphism.  Proposition 5.6.18 Let K be a difference field with a basic set σ, M a σoverfield of K, and H is a normal subgroup of Galσ (M/K). Let L be the fixed field of H (that is, L = {a ∈ M | φ(a) = a for every φ ∈ H}). Then

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(i) L/K is a σ-field extension which is σ-stable in M . (ii) The group consisting of σ-K-automorphisms of L which are contractions of σ-K-automorphisms of M is a homomorphic image of Galσ (M/K)/H. PROOF. Obviously, L is a σ-subfield of M containing K. Since the subgroup H is normal, for any λ ∈ Galσ (M/K) and φ ∈ H, one has λ−1 φλ ∈ H. Therefore, for every a ∈ L, λ−1 φλ(a) = a, hence φλ(a) = λ(a), so that λ(a) ∈ L. Thus, L/K is σ-stable in M . By Proposition 5.6.17, Galσ (M/L) is a normal subgroup of Galσ (M/K) and Galσ (M/K)/Galσ (M/L) is isomorphic to the group of all σ-K-automorphisms of L which are contractions of σ-K-automorphisms of M . Since H⊆Galσ (M/L),  Galσ (M/K)/Galσ (M/L) is a homomorphic image of Galσ (M/K)/H. Definition 5.6.19 Let K be an inversive ordinary difference field with a basic ˆ be a field extension of K. ˆ Then the set of all σ ∗ -field set σ = {α} and let L ˆ extensions of K defined on L is called a set of coordinate extensions of L/K (or ˆ If L is a σ ∗ -overfield of K, a set of coordinate extensions of K defined on L). ˆ is then an inversive difference field extension of K with the underlying field L said to be an extension of K coordinate with L/K. Example 5.6.20 As in Example 5.1.2, let us consider Q as an ordinary difference (σ-) field with the identity translation α, and let L and M be two σ-overfields of Q with the same underlying field Q(i) (i2 = −1) and translations that send i to i and −i, respectively. Then it is easy to see that the set of coordinate extensions of L/Q consists of this extension and the extension M/Q. The following statement and its corollary are direct consequences of the definitions of a coordinate extension and induced automorphism. We leave the proof to the reader as an exercise. Proposition 5.6.21 Let K be an inversive ordinary difference field with a basic ¯ the automorphism of set σ = {α}, L a σ ∗ -overfield of K, G = Gal(L/K), and α G induced by α (α ¯ : g → α−1 gα). Then the extensions coordinate with L/K are precisely those defined by the translations from the set αG = Gα. Furthermore, ¯.  if θ ∈ G, then the automorphism αθ of G induced by αθ is equal to θ¯α Corollary 5.6.22 With the notation of Proposition 5.6.21, let Aut(L) be the ˆ Then, if there are inversive difference group of all automorphisms of the field L. ˆ then the translations of these extensions field extensions of K defined on L, form a left coset and also a right coset of G in Aut(L) (and, hence, lie in the normalizer of G in Aut(L)). Furthermore, the automorphisms of the group G induced by these translations form the left and a right coset of the group of inner automorphisms of G in the group of all automorphisms of G.  Exercise 5.6.23 With the notation of Proposition 5.6.21, show that if θ and ρ are elements of the group G such that the translations αθ and αρ define ˆ then the corresponding difference isomorphic σ ∗ -extensions of K on the field L,

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ˆ αθ) → (L, ˆ αρ) should be an element of G satisfying the isomorphisms φ : (L, relationship α(φ)θ ¯ = ρφ. Conversely, prove that if the last equality holds for ˆ αθ) into (L, ˆ αρ). some element φ ∈ G, then φ is a difference isomorphism of (L, Note that it is possible that no element of G satisfies the above relationship (for example, this is the case if α and θ are identity mappings while ρ(x) = x for some x ∈ L). It is easy to see that every element φ ∈ G = Gal(L/K) defines a difference ˆ αφ) onto one of the coordinate extensions isomorphism of the difference field (L, ˆ of K defined on L. Therefore, there is one-to-one correspondence between difference K-isomorphisms of L/K onto coordinates extensions of L/K and elements of the group Gal(L/K). Because of the results of Theorems 5.1.6, 5.4.15, and 5.4.22, the study of compatibility of difference field extensions can be reduced to the study of compatibility of finite separable difference field extensions. In this connection, the following result seems to be quite important. Proposition 5.6.24 Let K be an inversive ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K such that the field extension L/K is finite and separable. Furthermore, let N be a σ-overfield of L whose underlying ˆ over K. ˆ Then field is a normal closure of L (i) The σ-field L is inversive. (ii) If L/K is compatible with every extension coordinate with N/K, then L/K is compatible with every σ-field extension of K. PROOF. Statement (i) is evident: since α(L) : K = α(L) : α(K) = L : K, one has α(L) = L. In order to prove (ii), notice, first, that N is a σ ∗ -field extension of K ˆ = K(a) ˆ such that N/K is finite, normal, and separable. Let N and let the ˆ minimal polynomial f (y) = Irr(a, K) be treated as a σ-polynomial of one σindeterminate y over K. Let M be any σ-field extension of K. Then f (y) ∈ M {y} and we can apply the existence theorem for ordinary difference polynomials (see Theorem 7.2.1 below), whose proof is independent of the material of this section and any results used in this section, to obtain that there is a solution b of the difference polynomial p(y) in some σ-field extension of M . Then there is a difference K-isomorphism of Kb onto an extension of K coordinate with N/K. Furthermore, by our assumption, there is a σ-K-isomorphism φ of L into a σ-overfield Ω of Kb. Since the field extension Kb/K is normal and Kb ˆ φ maps L into Kb and, hence, into contains a subfield K-isomorphic to L, M b. Thus, the difference field extensions L/K and M/K are compatible.  Let L be a finite normal separable σ-field extension of an inversive ordinary difference field K with a basic set σ = {α}. (As we have seen, such extensions are of primary importance in the analysis of compatibility.) It follows from Proposition 5.6.21 that L/K is one of the n coordinate extensions L1 /K = L/K, . . . , Ln /K where n = L : K. We divide these extensions into equivalence classes by placing two extensions Li /K and Lj /K in the same class if and only if they are σ-isomorphic (that is, there is a difference K-isomorphism of Li

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onto Lj ). Let us denote these equivalence classes by K1 , . . . , Kr with L/K ∈ K1 . Let ki and hi denote, respectively, the number of extensions in the class Ki and Card Galσ (L/K) for each extension L/K in Ki , i = 1, . . . , r. (We use the notation Galσ (L/K) for the set of all difference K-automorphisms of L no matter what extension of α to the underlying field of L is considered as a translation of L.) Furthermore, for every j = 1, . . . , n, let Gj = Galσj (Lj /K), dj = Card Galσj (Lj /K), αj the translation of Lj (which is an extension of α), and α ¯ j the automorphism of the group G = Gal(L/K) induced by αj . (clearly, if Lj /K ∈ Ki , then dj = hi .) Example 5.6.25 As in Example 5.1.11, let Q be considered as an inversive ordinary difference field with the identity translation α, let a and i denote the positive fourth root of 2 and the square root of −1, respectively, and let the field L = Q(i, a) be treated as a σ ∗ -overfield of Q (where the translation is also denoted by α) such that α(i) = −i and α(a) = −a. Furthermore, let K = (Q(i), α). Then one can easily see that the set of coordinate extensions of L/K consists of four elements: L1 = L = (Q(i, a), γ1 ), L2 = (Q(i, a), γ2 ), L3 = (Q(i, a), γ3 ), and L4 = (Q(i, a), γ4 ) where γ1 = α, and γ2 , γ3 , and γ3 are defined by the conditions γ2 (a) = a, γ3 (a) = ia, and γ4 (a) = −ia, respectively. Each of these extensions has two difference automorphisms, the identity and γ32 (see Example 5.6.13). There are two classes of coordinate extensions in this case: K1 = {L1 /K, L2 /K} and K2 = {L3 /K, L4 /K}. Theorem 5.6.26 Let K be an inversive ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K such that the field extension L/K is finite, normal, and separable. Let n = L : K. Then, with the notation introduced before Example 5.6.25, one has (i) n = hi ki for i = 1, . . . , r, n 1 (ii) r = di , n i=1 r  1 (iii) = 1. h i=1 i

PROOF. If an extension L /K belongs to a class Ki (1 ≤ i ≤ r), then the number of difference K-isomorphisms of L onto some extension L of the same class is equal to Card Galσ (L /K) = hi . It is easy to see that Gal(L/K) is the disjoint union of ki sets of such difference K-isomorphisms (where L runs trough the class Ki ). Therefore, hi ki = Card Gal(L /K) = n. For every i = 1, . . . , r, let Φi denote the set {j | Lj /K ∈ Ki }. Then ki hi = r  n    dj , hence rn = dj = dj . Φi

i=1 Φi

j=1

In order to prove statement (iii) one just needs to substitute ki = equality

r  i=1

ki = n.

n in the hi 

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Corollary 5.6.27 With the assumptions of the preceding theorem, either none of the extension coordinate with L/K is monadic or all are monadic. If they all are monadic, then they and all their difference subextensions are compatible with every difference field extension of K. If there are more than one coordinate extensions of L/K and they are not monadic, then each is incompatible with at least one of the other coordinate extensions. PROOF. Since L/K is normal, all coordinate extensions are strongly σstable. It follows that the extensions of class Ki are monadic if and only if hi = 1. Applying statement (iii) of the last theorem we obtain that one of these coordinate extensions is monadic if and only if there is but one class. Then they are all monadic an Proposition 5.6.24 implies that the coordinate extensions and all their subextensions are compatible with very difference field extension of K. In order to prove the last statement, notice that two coordinate extensions are compatible if and only if they are isomorphic. Indeed, because of the normality of L/K, for every difference overfield M of K and for every difference isomorphisms φ : Li /K → M/K and ψ : Lj /K → M/K, one has φ(Li ) = ψ(Lj ). Therefore, φ−1 ψ is a difference K-isomorphism of Lj onto Li . If there are more than one coordinate extensions and they are not monadic, then, as we have seen, r ≥ 2, and each coordinate extension of some class is incompatible with the coordinate extensions of another class.  Definition 5.6.28 Let K be an ordinary difference field with a basic set σ = {α} and L a σ-field extension of K such that L/K is normal and separable. The extension L/K is called isolated if the induced automorphism α ¯ of the group Gal(L/K) is identity, that is, Galσ (L/K) = Gal(L/K). The following statement is a direct consequence of Corollary 5.6.22. Proposition 5.6.29 Let L/K be a normal separable ordinary difference field extension with a basic set σ = {α}. Then the set of extensions coordinate with L/K contains an isolated extension if and only if α ¯ is an inner automorphism of Gal(L/K), and these extensions are then all isolated if and only if the group Gal(L/K) is commutative.  Proposition 5.6.30 With the assumptions of Proposition 5.6.29 and n = Card Gal(L/K), we have the following statements. (i) If n is prime, then the coordinate extensions of L/K are either all monadic or all isolated and mutually incompatible. (ii) If n = pq, where p and q are distinct primes, then there are just the following four possibilities: (a) All coordinate extensions Li (1 ≤ i ≤ n) of L/K are monadic. (b) All coordinate extensions of L/K are isolated and mutually incompatible. (c) There are p classes of coordinate extensions Li /K such that Card Gal(Li /K)=p or there are q classes of coordinate extensions Li /K with Card Gal(Li /K) = q.

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(d) There is one isolated coordinate extension, a classes of coordinate extensions Li /K with Card Gal(Li /K) = p, and b classes of coordinate extensions Lj /K with Card Gal(Lj /K) = q, where a and b are solutions of the Diophantine equation qx + py = n − 1. PROOF. Statement (i) is a direct consequence of Theorem 5.6.26(i). Suppose that there are no monadic or isolated extensions among the coordinate extensions L1 , . . . , Ln . Let a be the number of classes of coordinate extensions Li /K with Card Gal(Li /K) = p, and let b be the number of classes a b of extensions Lj /K with Card Gal(Lj /K) = q. By Theorem 5.6.26, + = 1, p q or aq + bp = pq. Then p|a, q|b and we may write pqm + pqm = pq for some positive nonnegative m and m . Then m + m = 1 hence either m = 0 or m = 0, that is, either a = 0, b = q, or a = p, b = 0. Now suppose that there is an isolated coordinate extension of K. It is easy to see that the number of isolated extensions coordinate with L/K is the order of the center Z of the group Gal(L/K). It follows from the Sylow theorems that every group of order pq is either commutative (assuming that p ≤ q, this is the case if p = q, or p < q and p q) or has the trivial center. Therefore, either all coordinate extensions Li /K are isolated or just one is. In the latter case, with the same a and b as before, Theorem 5.6.26(iii) implies that 1 a b + + = 1, p q pq hence aq + bp = n − 1. It is easy to see that the last equations has just one positive integer solution with respect to a and b.  In what follows we are considering finitely generated monadic extensions. In particular, it will be shown that if such an extension is separable, then it has limit degree one. We start with the following result by R. Cohn called in [41] a Mapping Theorem. Theorem 5.6.31 Let K be an inversive ordinary difference field with a basic set σ and let L be a σ ∗ -overfield of K such that the field extension L/K is normal and separable. Let G = Gal(L/K) and for any φ ∈ G, let Lφ denote the difference field with the underlying field L and the basic set σφ = {αφ}. Furthermore, let F/K be a σ ∗ -subextension of L/K and let H = Gal(L/F ). Then (i) The difference K-isomorphisms of F into Lφ (i. e., K-isomorphisms µ : F → Lφ such that µ(α(a)) = αφ(µ(a)) for every a ∈ F ) are the contractions to F of the K-automorphisms θ ∈ G such that α ¯ (θ) ∈ φθH. (ii) Two mappings θ1 , θ2 ∈ G satisfying the above condition yield the same difference K-isomorphism of F into Lφ if and only if θ1 and θ2 lie in the same coset of H in G.

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(iii) The σ ∗ -field extension F/K is monadic if and only if for any θ ∈ G, the inclusion α ¯ (θ) ∈ HθH implies θ ∈ H. (iv) If the field extension L/K is finite, then the extension F/K is σ-stable if and only if for any θ ∈ G, the inclusion α ¯ (θ) ∈ GθG implies that θ lies in the normalizer of H. PROOF. Since the field extension L/K is normal, every K-automorphism of F into L extends to a K-automorphism of K-automorphism of L (see Theorem 1.6.8(v)). Thus, every K-automorphism of F into Lφ (φ ∈ G) is a contraction of some automorphism from G. On the other hand, if θ ∈ G, then the contraction of θ to F is a difference K-isomorphism of F into Lφ if and only if for every a ∈ F , ¯ (θ)(a) = a, that is, if and only if θ−1 φ−1 α ¯ (θ) ∈ H θα(a) = αφθ(a) or θ−1 φ−1 α or α ¯ (θ) ∈ φθH. Statement (ii) of the theorem is obvious. Suppose that F/K is monadic and there is an element θ ∈ G \ H such that α ¯ (θ) ∈ HθH, that is, α ¯ (θ) = λθµ for some λ, µ ∈ H. By part (i) of our theorem, the contraction of θ to F is a non-identical difference K-isomorphism of F into Lλ . Since the identity mapping is also a difference K-isomorphism of F into Lλ (¯ α(e) = e ∈ λeH = H), the extension F/K is not monadic, contrary to our assumption. Suppose now that F/K is not monadic. Let θ1 and θ2 be elements of G whose contractions to F are distinct difference K-isomorphism of F into some / H, α(θ) ¯ 1 = ψθ1 λ, and α(θ) ¯ 2 = ψθ2 µ where Lψ , ψ ∈ G. Then θ = θ1−1 θ2 ∈ λ, µ ∈ H. It follows that α(θ) ¯ = (ψθ1 λ)−1 ψθ2 µ = λ−1 θ1−1 θ2 µ ∈ HθH. This contradiction completes the proof of statement (iii). In order to prove the last statement of the theorem, suppose that an element θ ∈ G does not belong to the normalizer of H and α ¯ (θ) = γ1 θγ2 for some γ1 , γ2 ∈ H. Since for any a ∈ F we have θ(a) = αα ¯ (θ)(a) = αγ1 θγ2 (a) = (αγ1 )θ(a), the contraction of θ to F is a σ-K-isomorphism of F into Lγ1 which does not leave the field F fixed. Since γ1 ∈ H, Lγ1 is a difference overfield of F and we obtain that the extension F/K is not σ-stable. Conversely, suppose that there is a difference K-isomorphism ρ of F into some Lφ , which is a difference overfield of F , such that ρ(F ) = F . Then φ ∈ H and ρ does not lie in the normalizer of H. By part (i) of our theorem, we have α ¯ (ρ) = φρχ for some χ ∈ H. This completes the proof.  Corollary 5.6.32 Let a difference (σ-) field extension L/K satisfy the assumptions of the last theorem. If the set of extensions coordinate with L/K contains an isolated extension, then L/K has no properly monadic subextensions. PROOF. It follows from Proposition 5.6.29 that α ¯ is an inner automorphism of the group G = Gal(L/K). Let φ be an element of G that induces this automorphism. Suppose that F/K is a monadic σ-subextension of L/K. Without loss of generality we may assume that the σ-field F is inversive. Let H = Gal(L/F ). Then for any θ ∈ G, the inclusion φ−1 θφ ∈ HθH implies that θ ∈ H. Setting θ = φ we obtain that φ ∈ H, hence φ−1 λφ ∈ HλH for every λ ∈ G. It follows

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that H = G. Let us show that the last equality implies that F = K. Indeed, if a ∈ F and b is a conjugate of a in L, then there is a K-isomorphism χ of K(a) onto K(b) such that χ(a) = b. By Corollary 1.6.41, χ extends to an isomorphism of L into an overfield of L. Since the field extension L/K is normal, χ ∈ G. Therefore, b = a, so the element a has no conjugates in L, except for a itself. Since L/K is normal and separable, we obtain that a ∈ K. Thus, L/K has no properly monadic subextensions.  Theorem 5.6.33 Let K be an ordinary difference field with a basic set σ and let L be a σ-overfield of K such that the field extension L/K is algebraic. Furthermore, let S denote the separable closure of K in L. Then L/K is monadic if and only if S/K is monadic. PROOF. Suppose, first, that L/K is monadic. If S/K is not monadic, then there is a σ-K-isomorphism φ, distinct from the identity, of S into some σoverfield M of S. By Theorem 5.4.22, L/S is compatible with every σ-field extension of S, so that we may assume that L ⊆ M . Furthermore, since LS = S, φ can be extended to a σ-K-isomorphism φ of L into a σ-overfield M  of M . Since φ is not identity homomorphism, we obtain a contradiction with the monadicity of L/K. Conversely, if S/K is monadic, then the field extension L/S is purely inseparable, hence it has at most one isomorphism into any overfield of S (see Proposition 1.6.21(ii)). It follows that the extension L/S is monadic, hence L/K is monadic.  Theorem 5.6.34 Let K be an inversive ordinary difference field with a basic set σ = {α}, L a σ-overfield of K, and L∗ the inversive closure of L. Furthermore, suppose that L∗ = KA for some set A such that (a) The field extension K(A)/K is normal and separable. (b) If Aσ denotes the set {αi (a) | a ∈ A, i ∈ Z, i = 0}, then the fields K(A) and K(Aσ ) are linearly disjoint over K. Then the σ-field extension L∗ /K has no properly monadic σ-subextensions. PROOF. For every i ∈ Z, let Ai = {αi (a) | a ∈ A}, Aσi = {αi (a) | a ∈ Aσ }, and Gi = Gal(K(Ai )/K). It follows from Theorem 1.6.43 that whenever g1 ∈ Gi1 , . . . , gk ∈ Gik (k ≥ 1, i1 , . . . , ik ∈ Z), there is a unique automorphism g ∈ Gal(L∗ /K) whose restriction on each Gij (1 ≤ j ≤ k) is gj . Furthermore, condition (a) of our theorem implies that for any g ∈ Gal(L∗ /K) and for any i ∈ Z, the contraction of g on K(Ai ) is a K-automorphism of K(Ai ). It follows that the group Gal(L∗ /K) is isomorphic to the direct sum i∈Z Gi . Furthermore, it is easy to see that if g ∈ G0 = Gal(K(A)/K), then αi gα−i belongs to Gi and the mapping φi : G0 → Gi defined by g → αi gα−i is a group isomorphism. Let us index the elements of G0 with indices from some set J (so that G0 = {gj }j∈J ) and write gij for φi (gj ) (i ∈ Z, j ∈ J). Then Gi = {gij }j∈J for any i ∈ Z, and elements of the group Gal(L∗ /K) have unique representations as strings (. . . , g−1j−1 , g0j0 , g1j1 , . . . ) where . . . , j−1 , j0 , j1 , · · · ∈ J.

(5.6.1)

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Let α ¯ denote the inner automorphism of the group Gal(L∗ /K) induced by α (that is, α(g) ¯ = α−1 gα for every g ∈ Gal(L∗ /K)). Let us show that if an element g ∈ Gal(L∗ /K) is represented in the form (5.6.1), then α ¯ (g) = (. . . , g−2j−1 , g−1j0 , g0j1 , . . . ) (so that α(g) ¯ is obtained from g by moving each coordinate one place to the left). Indeed, it is clear that one just needs to prove this property for the case when g belongs to some Gi . Then for any k ∈ Z and x ∈ Ak we have α ¯ (g)(x) = α−1 gα(x) = α−1 gk+1,jk+1 α(x) = α−1 αk+1 gjk+1 α−k−1 α(x) = αk gjk+1 α−k (x) = gkjk+1 (x). It follows that gkjk+1 is the kth component of α(g). ¯ Let us show that if F/K is a proper σ-subextension of L/K, then F/K is not monadic. Without loss of generality we can assume that the σ-field F is inversive. Let H = Gal(L∗ /F ). We are going to prove that there exists g ∈ Gal(L∗ /K)\H such that α ¯ (g) ∈ gH. By Theorem 5.6.31, this implies that there is a difference K-isomorphism of F into L∗ which is not the identity mapping, so the extension F/K is not monadic. First of all, notice that there is p ∈ N and elements θ0 , . . . , θp ∈ G such that the elements of G of the form (. . . , θ00 , . . . , θpp , . . . ) do not belong to H for any choice of unspecified coordinates. Indeed, since the extension F/K is proper, one can choose an element
r x i∈ F \ K. Since L = KA, there exist k, r ∈ N K( such that αk (x) ∈ i=0 α (A)). Furthermore, there is a K-automorphism
r λ of the
field K( i=0 αi (A)) which maps αk (x) to a different element. Since r Gal(K( i=0 αi (A))/K) is the direct sum of the groups G0 , . . . , Gr , there exist elements λ0 , . . . , λr ∈ G such that λ = λ00 . . . λrr . Setting p = r and θi = λi for i = 0, . . . , p, we obtain the elements θ0 , . . . , θp ∈ G which satisfy the desired condition. Considering finite sets of elements of G with the above property, let us choose such a set {θ0 , . . . , θp } with the smallest possible p. Let Θ denote the corresponding set of all elements of the form (. . . , θ00 , . . . , θpp , . . . ), and let Θ = ¯ (g) | g ∈ Θ}. {g −1 α Let µj = θj−1 θj+1 (j = 0, . . . , p − 1). It is easy to check that Θ is the set of elements of the form (. . . , µ00 , . . . , µp−1,p−1 , . . . ) for all possible choices of unspecified coordinates. By the minimality of p, there exists and element  ν ∈ Θ H. (If p = 0, then Θ = G and such an element is identity.) Let g be ¯ (g) = ν. Then g ∈ / H and g −1 α(g) ¯ ∈ H.  an element of Θ such that g −1 α Theorem 5.6.35 Let K be an inversive ordinary difference field with a basic set σ = {α} and let L a finitely generated monadic σ-field extension of K. Then the field extension L/LK is purely inseparable. In particular, if the field extension L/K is separable, then LK = L. PROOF. Suppose, first, that L/K is a subextension of a the inversive closure of a benign σ-field extension M/K with minimal standard generator a. Then the set A = {a} satisfies the requirements of Theorem 5.6.34. Indeed, if it is not

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so, then, setting n = K(a) : K, we obtain that there exist positive integers j and k such that K(α−j (a), . . . , αk (a)) : K(α−j (a), . . . , α−1 (a), α(a), . . . , αk (a)) < n. Since σ-field K is inversive, one has K(αi (a)) : K = n for every i ∈ Z, hence K(α−j (a), . . . , αk (a)) : K < nj+k+1 . The last inequality, in turn, implies the inequality K(a, . . . , αj+k (a)) : K < nj+k+1 which contradicts the condition ld(Ka/K) = n. Therefore, in this case we have L = K. Let us consider the general case. Let S denote the separable closure of K in L (treated as a σ-subfield of L). Then Theorem 5.6.33 implies that the σfield extension S/K is monadic. Let N denote the normal closure of S over K. By Theorem 5.4.13, there exists a chain of σ-field extensions K ⊆ N0 ⊆ N1 ⊆ . . . Nm such that N0 = NK = (Nm )K , each σ-field Ni is inversive, each extension Ni /Ni−1 is equivalent to a benign extension, and Nm /K is equivalent to N/K. It is easy to see that if j is the smallest integer such that S ⊆ Nj , then j = 0. Indeed, if j > 0, then the extension Nj−1 S/Nj−1 is a monadic subextension of Nj /Nj−1 . As we have shown, in this case Nj−1 S = Nj−1 , so that one has the inclusion S ⊆ Nj−1 which contradicts our choice of j. Thus, S = LK hence  the extension L/LK is purely inseparable. The following two statements are direct consequences of the last theorem. We leave the proof to the reader as an exercise. Corollary 5.6.36 Let K be an inversive ordinary difference field with a basic set σ and let L a finitely generated monadic σ-field extension of K. Then rld(L/K) = 1. In particular, if Char K = 0, then ld(L/K) = 1.  Corollary 5.6.37 If an inversive difference field of zero characteristic admits a finitely generated proper monadic extension, then it admits a proper monadic extension of finite degree.  Theorem 5.6.38 Let C(x) denote the field of rational functions in one variable x with coefficients in the field of complex numbers. Let K denote the ordinary difference field with the underlying field C(x) and basic set σ = {α} where α(f (x)) = f (x + 1) for every f (x) ∈ C(x). Then (i) K has no incompatible difference field extensions. (ii) The field K admits no finitely generated proper monadic extensions. PROOF. Statement (ii) is an immediate consequence of Corollary 5.6.37. To prove part (i), suppose that g is an element of some σ-overfield of K, and let

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g be algebraic over C(x). If g ∈ / K, then g has finite branch points c1 , . . . , ck . Then αi (g) (i = 0, 1, 2, . . . ) has finite branch points c1 + i, . . . , ck + i. Therefore, for any positive integer r, αr (g) has at least one branch point which / is not among the branch points of g, α(g), . . . , αr−1 (g). Therefore, αr (g) ∈ C(x, g, α(g), . . . , αr−1 (g)) hence ld(Kg/K) > 1. It follows from Theorem 5.4.22 that K has no incompatible σ-field extensions.  The following result gives a similar characterization of another difference field with the underlying field C(x). We refer the reader to [41, Chapter 9, Theorem XX] for the proof. Theorem 5.6.39 Let q be a nonzero complex number such that q n = 1 for n = 1, 2, . . . . Let F be the ordinary difference field with the underlying field C(x) and basic set σ = {α} where the translation α is defined by the condition α(f (x)) = f (qx) for every f (x) ∈ C(x). Then (i) A σ-field extension L/F is incompatible with some other σ-field extension of F if and only if L contains a kth root of x for some positive integer k. (ii) F admits no finitely generated proper monadic extensions. Theorem 5.6.40 Let K be an inversive ordinary difference field with a basic set σ and let L = KA where the set A is σ-algebraically independent over K. Furthermore, let L∗ be the inversive closure of L. Then (i) If M is a finitely generated separably algebraic monadic σ-field extension of L∗ , then there exists a finitely generated monadic σ-field extension F/K such that M = L∗ F . (ii) If F is any monadic σ-field extension of K and M = L∗ F , then the extension M/L∗ is monadic. PROOF. If M/L∗ is a finitely generated separably algebraic monadic σ-field extension, then Theorem 5.6.35 implies that ML∗ = M . It follows from Theorem 4.3.19 that the difference field M , which is a core over an inversive σ-field, is inversive. Applying Theorem 5.4.25 we obtain that the σ-fields M and L∗ MK  have the same inversive closure, hence M = L∗ MK . It follows from Corollary 5.6.36 and Proposition 4.3.12 that M : L∗ < ∞, hence there is a finite set B ⊆ MK such that M = L∗ (B). It is easy to see that MK = K(B). Let us denote this σ-field by F . Then the set A is σ-algebraically independent over F and we obtain σ-field extensions M/L∗ , L∗ /K, and F/K such that M/L∗ is monadic, M = L∗ F , and L∗ is the inversive closure of KA where the set A is σ-algebraically independent over F . On the other hand, if we have a monadic σ-field extension F/K with F, M, K, and L∗ satisfying assumptions of part (ii) of the theorem, then the field extension F/K is algebraic, hence the set A is σ-algebraically independent over F , L∗ is the inversive closure of KA, and M = L∗ F . Thus, both parts of the theorem will be proven if we show that if F/K is a σ-field extension, a set A is σ-algebraically independent over F , and L∗ is the inversive closure of KA, then L∗ F /L∗ is monadic if and only if F/K is

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monadic. But this is obvious, since any σ-K-isomorphism of F naturally extends to a σ-L∗ -isomorphism of L∗ F , so if two σ-L∗ -isomorphisms of L∗ F  coincide on F , they coincide on A and therefore they are equal.  The following result gives a characterization of finite difference field extensions of an ordinary difference (σ-) field K which are compatible with every other σ-field extension of K. In section 8.1 we will return to the study of such “universally compatible” extensions in more general case. Theorem 5.6.41 Let K be an inversive ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K such that the field extension L/K is finite. Then L/K is compatible with every difference field extension of K if and only if L/K is monadic. PROOF. First of all, without loss of generality we may assume that K is inversive. Furthermore, it follows from Theorems 5.1.6 and 5.6.33 that one may suppose that the extension L/K is separable. Let N be the normal closure of L over K. Then we may consider N as a finitely generate separably algebraic normal σ-field extension of K of limit degree 1. By Proposition 5.6.24(ii), the extension L/K is compatible with every difference field extension of K if and only if L/K is compatible with every extension coordinate with N/K. Let G = Gal(N/K) and let H = Gal(N/L). By Theorem 5.6.31, there is a difference isomorphism of L/K into every extension coordinate with N/K if and ¯ (γ) ∈ φγH. Let us associate only if for every φ ∈ G there exists γ ∈ G such that α ¯ (θ)Hθ−1 . Note, that with every coset A = θH of H in G (θ ∈ G) the set A = α H is a σ-stable subgroup of G (see Proposition 5.6.11), so A does not depend on the choice of a representative of the coset A. Furthermore, Card A = Card H and if β ∈ A, then α(β) ¯ ∈ φβH if and only if φ ∈ A . Therefore, the condition that there is a difference isomorphism of L/Kinto every extension coordinate with N/K is equivalent to the condition G = {A | A is a coset of H in G}. Since the group G is finite and Card A = Card H the last condition is equivalent to the requirement that all sets A are disjoint, which, obviously, equivalent to the following condition: (∗) For any elements θ, β ∈ G, the inclusions α(θ) ¯ ∈ φθH and α(β) ¯ ∈ φβH imply β ∈ θH. Let us show that the last condition is equivalent to the condition of Theorem 5.6.31(iii): L/K is monadic if and only if for any θ ∈ G, the inclusion α(θ) ¯ ∈ HθH implies θ ∈ H. Indeed, suppose that the condition of Theorem 5.6.31(iii) ¯ = φβγ2 for some γ1 , γ2 ∈ H. Then α(θ ¯ −1 β) = holds and let α(θ) ¯ = φθγ1 , α(β) −1 −1 −1 γ1 θ βγ2 , hence θ β ∈ H hence β ∈ θH. Conversely, suppose that condition (∗) holds. Let α ¯ (θ) = γ1 θγ2 for some ¯ ∈ γ1 θH. Since α ¯ (e) ∈ γ1 eH, where e is the identity of γ1 , γ2 ∈ H. Then α(θ) the group G, we have θ ∈ eH = H. This completes the proof.  Theorem 5.6.42 Any subextension of a finitely generated monadic ordinary difference field extension is monadic.

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PROOF. Let K be an ordinary difference field with a basic set σ, let L be a σ-overfield of K such that the difference field extension L/K is finitely generated and monadic, and let F be an intermediate difference (σ-) field of L/K. Because of the results of Proposition 5.6.4(i) and Theorem 5.6.33, while proving that the extension F/L is also monadic, we can assume that K is inversive and the extension L/K is separable. With these assumptions, it follows from Theorem 5.6.35 that L is a finite extension of the field K. By Theorem 5.6.41, the extension L/K is compatible with every difference field extension of K. Then F/K is also compatible with every σ-field extension of K. Applying Theorem 5.6.41 once again we obtain that F/K is monadic.  Note that the condition that L/K is finitely generated is essential in the last theorem. Furthermore, there are finitely generated monadic ordinary difference field extensions which are not subextensions of normal monadic extensions. The corresponding examples are due to R. Cohn (see [41, Chapter 9, Example 5]). Exercise 5.6.43 Let K be an algebraically closed ordinary difference field. Prove that any two difference field extensions of K are compatible, and K has no properly monadic extension. Exercise 5.6.44 Let K be an inversive ordinary difference field with a basic set σ. Prove that the following two conditions are equivalent. (i) K admits incompatible σ-field extensions or K has a finitely generated properly monadic extension. (ii) There is a σ-field extension L/K, L = K, of finite separable degree. [Hint: Use Theorems 5.4.22 and 5.6.35.] Exercise 5.6.45 Let K be an ordinary difference field which is not algebraically closed. Prove that if every element of K is periodic, then K admits either incompatible extensions or a properly monadic extension. Exercise 5.6.46 Let K be an inversive ordinary difference field with a basic set σ and let L = KS where the set S is σ-algebraically independent over K. Furthermore, let L∗ denote the inversive closure of L. Prove that (i) The σ-field K admits incompatible extensions if and only if L∗ admits such extensions. (ii) There is a finitely generated separable properly monadic extension of K if and only if there is such an extension of L∗ .

Chapter 6

Difference Kernels over Partial Difference Fields. Difference Valuation Rings 6.1

Difference Kernels over Partial Difference Fields and their Prolongations

In this section we extend the study of difference kernels presented in Section 5.2 to the case of partial difference fields. The corresponding theory is due to I. Bentsen [10]. Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }. Let us fix an index i ∈ {1, . . . , n} and let K (i) denote the field K treated as an inversive difference field with a basic set σ \ {αi } denoted by σi . Then αi can be viewed as a σi -automorphism of K (i) . (Also, in accordance with our basic conventions, K (i) S and K (i) S∗ will denote, respectively, the σi - and σi∗ - field extension of K (i) generated by a set S.) Definition 6.1.1 With the above notation (and a fixed index i, 1 ≤ i ≤ n), a difference (σ-) kernel R over K is an ordered pair (K (i) a0 , . . . , ar , τ ) where K (i) a0 , . . . , ar  is a σi -field extension of K (i) generated by s-tuples a0 , . . . , ar (1) (s) (s ≥ 1 and each aj is an s-tuple (aj , . . . , aj ) with coordinates in some σi -overfield of K) and τ is an extension of αi to a σi -isomorphism of the field (k) (k) K (i) a0 , . . . , ar−1  onto K (i) a1 , . . . , ar  such that τ (aj ) = aj+1 ( 0 ≤ j ≤ r−1, 1 ≤ k ≤ s). The number r is said to be the length of the kernel R. (If r = 0, we mean that τ is the σi -automorphism αi of K (i) .) Note that if the kernel R is of length 0 or 1, then the expressions K (i) a0 , . . . , ar−1  and K (i) a1 , . . . , ar−1 , respectively, are interpreted as K (i) . 371

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Definition 6.1.2 With the notation of Definition 6.1.1, a prolongation of a difference kernel R over K is a σ-kernel R consisting of a σi -overfield (1) (s) K (i) a0 , . . . , ar , ar+1  of K (i) a0 , . . . , ar  (ar+1 is an s-tuple (aj , . . . , aj ) over (k)

(k)

K (i) ) together with an extension τ  of τ such that τ  (ar ) = ar+1 , 1 ≤ k ≤ s. A prolongation R of R is called generic if the inversive closure K (i) a0 , . . . , ar+1 ∗ of the σi -field K (i) a0 , . . . , ar+1  is a free join of the σi∗ -fields K (i) a0 , . . . , ar ∗ and K (i) a1 , . . . , ar+1 ∗ over K (i) a1 , . . . , ar ∗ . Notice that the definition of a generic prolongation is not ambiguous (see Propositions 2.1.7 and 2.1.8(ii)). Furthermore, if R is a prolongation of R such that K (i) a0 , . . . , ar  and K (i) a1 , . . . , ar+1  are algebraically disjoint over the field K (i) a1 , . . . , ar , then statements (ii) and (iv) of Proposition 2.1.8 imply that R is a generic prolongation of R. Definition 6.1.3 With the above notation, we say that a difference kernel R satisfies property P if there exists a σi -overfield L1 of K (i) a0 , . . . , ar , a σi -subfield L of L1 which contains K (i) a0 , . . . , ar−1 , and an extension of τ to a σi -isomorphism τˆ of L into L1 such that (i) The field extension L1 /L is primary. (ii) L and K (i) a0 , . . . , ar  are algebraically disjoint over K (i) a0 , . . . , ar−1 . (iii) If r > 0, then L1 = Lˆ τ (L); if r = 0, then τˆ(L) ⊆ L. Definition 6.1.4 With the above notation, a kernel R is said to satisfy property P∗ (with respect to (M, M1 , τ˜) ) if there exists an inversive σi -overfield M1 of K (i) a0 , . . . , ar , an inversive σi -subfield M of M1 which contains K (i) a0 , . . . , ar−1 , and an extension of τ to a σi -isomorphism τ˜ of M into M1 such that (i) The field extension M1 /M is primary. (ii) M and K (i) a0 , . . . , ar ∗ are algebraically disjoint over K (i) a0 , . . . , ar−1 ∗ . (iii) If r > 0, then M1 = M ˜ τ (M ); if r = 0, then τ˜(M ) ⊆ M . Proposition 6.1.5 The properties P and P∗ are equivalent. PROOF. Let L, L1 and τˆ be as in Definition 6.1.3 of property P, let M1 = L∗1 (the inversive closure of L1 ), and let M be the inversive closure of L in M1 . By Proposition 2.1.8 (parts (ii) and (iii) ), τˆ extends uniquely to a σi -isomorphism τ (M ), τ˜ of M into M1 such that if the length r of R is positive, then M1 = M ˜ and if r = 0, then τ˜(M ) ⊆ M by the monotonicity of the operation ∗ . Applying Proposition 2.1.8 (parts (ii) and (iv) ) we obtain that P implies P∗ . Now suppose that M , M1 and τ˜ are as in Definition 6.1.4 of property P∗ . Let L denote the separable part of M over K (i) a0 , . . . , ar−1 . Then L is a σi overfield of K (i) a0 , . . . , ar−1  such that L and K (i) a0 , . . . , ar  are algebraically disjoint over K (i) a0 , . . . , ar−1 . Let τˆ be the contraction of τ˜ to a σi -isomorphism of L into M1 . If r = 0 then for any b ∈ L, the element τ˜(b) is separably algebraic over τˆ(K (i) ) = K (i)

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373

and τˆ(b) ∈ M . Therefore, τˆ(L) ⊆ L. Let us set L1 = LK i a0  if r = 0 and τ (L) if r > 0. In either case L1 is a σi -overfield of K (i) a0 , . . . , ar−1  L1 = Lˆ and the extensions M/L and M1 /M are primary. By Theorem 1.6.48, M1 /L  and L1 /L are primary as well, so that P∗ implies P. Definition 6.1.6 With the above notation, we say that a difference kernel R satisfies property L∗ if it satisfies property P∗ strengthen by replacing the requirement that the fields L and K (i) a0 , . . . , ar  are algebraically disjoint over K (i) a0 , . . . , ar−1  by the requirement that these fields are quasi-linearly disjoint over K (i) a0 , . . . , ar−1 . Proposition 6.1.7 A difference kernel R = (K (i) a0 , . . . , ar , τ ) satisfies property L∗ if and only if the field extension K (i) a0 , . . . , ar ∗ /K (i) a0 , . . . , ar−1 ∗ is primary. PROOF. If the extension K (i) a0 , . . . , ar ∗ /K (i) a0 , . . . , ar−1 ∗ is primary, one can set M1 = K (i) a0 , . . . , ar ∗ , M = K (i) a0 , . . . , ar−1 ∗ and apply parts (ii) and (iii) of Proposition 2.1.8 to obtain that R satisfies property L∗ . Conversely, if the kernel R satisfies L∗ , then Corollary 1.6.50 immediately implies  that the extension K (i) a0 , . . . , ar ∗ /K (i) a0 , . . . , ar−1 ∗ is primary. Exercise 6.1.8 With the above notation, prove that if the extension K (i) a0 , . . . , ar ∗ /K (i) a0 , . . . , ar−1 ∗ is primary, the extension K (i) a0 , . . . , ar ∗ / K (i) a0 , . . . , ar−1 ∗ is also primary. Show that the converse statement also holds if the fields K (i) a0 , . . . , ar−1 ∗ and K (i) a0 , . . . , ar  are quasi-linearly disjoint over K (i) a0 , . . . , ar−1 . The following example presents a kernel which does not satisfy P∗ . Example 6.1.9 As in Example 5.1.17, let us consider Q as a difference field with a basic set σ = {α1 , α2 } where α1 and α2 are the identity automorphisms. Let b and i denote the positive square root of 2 and the square root of −1, respectively (b, i ∈ C), and let F = Q(b, i). We consider F as a σ-overfield of Q where the extensions of α1 and α2 to automorphisms of F (denoted by the same letters) are defined by their actions on b and i as follows: α1 (b) = −b, α1 (i) = i, α2 (b) = b, and α2 (i) = −i. Let F (2) denote the field F treated as an ordinary difference field with the basic set σ2 = {α1 } (we keep the notation of Definition 6.1.1). By Corollary 5.1.16, there exists an algebraic closure G of the field F that has a structure of a σ2 -overfield of F (2) . By Proposition 2.1.9(ii), the σ2 -field G is inversive. Furthermore, there exists an element a ∈ G \ F such that a2 = b. Let us consider the difference (σ2 -) kernel R = (F (2) a, τ ) where τ = α2 (this mapping is considered as a σ2 -automorphism of F (2) ). Suppose that this kernel satisfies P∗ . Then there exist inversive σ2 -overfields M1 and M of F (2) a and F (2) , respectively, such that the field extension M1 /M is primary. It follows that a ∈ M and there exists a σ2 -isomorphism of M into M which coincides with α2 on F (2) . Denoting this isomorphism by α2 , we can consider M as a

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σ-overfield of F which contains a. On the other hand, Example 5.1.17 shows that a is not contained in any σ-field extension of F . This contradiction implies that the kernel R does not satisfy P∗ . Theorem 6.1.10 (i) If a difference kernel R satisfies P∗ , then R has a generic prolongation which satisfies P∗ . (ii) If a generic prolongation R of a difference kernel R satisfies P∗ , then there exists a triple (M, M1 , τ˜) with respect to which R satisfies P∗ and trough which a generic prolongation R of R can be obtained such that R is equivalent to R in the sense of isomorphism. (iii) If a difference kernel satisfies L∗ , then all its generic prolongations are equivalent and satisfy L∗ . PROOF. Suppose that a difference (σ-) kernel R = (K (i) a0 , . . . , ar , τ ) satisfies P∗ with respect to the triple (M, M1 , τ˜) (see Definition 6.1.4).By Theorem 5.1.8, (with (M, M1 , τ˜) corresponding to (K, L, φ) in Theorem 5.1.8), there exists an inversive σ-overfield M2 of M1 and an extension τ˜1 of τ˜ such that M2 = τ1 (M1  and the field extension M2 /M1 is primary. Setting ar+1 = τ˜1 (ar ) M1 ˜ and denoting by τ  the contraction of τ˜1 to a σi -isomorphism of K (i) a0 , . . . , ar  to K (i) a1 , . . . , ar+1 , we obtain the prolongation R = (K (i) a0 , . . . , ar+1 , τ  ) of the kernel R. Since the fields M and K (i) a0 , . . . , ar ∗ are algebraically disjoint over (i) K a0 , . . . , ar−1 ∗ , τ˜1 (M ) and K (i) a1 , . . . , ar+1 ∗ are algebraically disjoint over K (i) a1 , . . . , ar ∗ . Also, the fact that M1 and τ˜1 (M1 ) are quasi-linearly disjoint, hence algebraically disjoint, over τ˜1 (M ) implies that the fields M1 and τ˜1 (M )ar+1 ∗ are algebraically disjoint over τ˜1 (M ). Applying Theorem 1.6.44(ii) we obtain that K (i) a0 , . . . , ar ∗ and K (i) a1 , . . . , ar+1 ∗ are algebraically disjoint over K (i) a1 , . . . , ar ∗ and also M1 and K (i) a0 , . . . , ar+1 ∗ are algebraically disjoint over K (i) a0 , . . . , ar ∗ . Therefore, R is a generic prolongation of R which satisfies P∗ . Now suppose that R satisfies L∗ . Then by Propositions 2.1.8 (parts (ii) and (iii)) and 6.1.7, R satisfies P∗ with M and M1 (see Definition 6.1.4) denoting K (i) a0 , . . . , ar−1 ∗ and K (i) a0 , . . . , ar ∗ , respectively. Applying parts (ii) and (iii) of Proposition 2.1.8 once again we obtain (with the notation of the first part of the proof) that K (i) a0 , . . . , ar+1 ∗ = M2 . It follows that R is a generic prolongation of R which satisfies L∗ . Suppose that R1 = (K (i) a0 , . . . , ar , b, θ) be another generic prolongation of R. then the inversive closure N of K (i) a0 , . . . , ar , b is the free join of K (i) a0 , . . . , ar ∗1 and K (i) a1 , . . . , ar , b∗1 over K (i) a1 , . . . , ar ∗1 where ∗1 denotes the inversive closure in N . Furthermore, by Proposition 2.1.7, K (i) a0 , . . . , ar ∗1 may be identified with K (i) a0 , . . . , ar ∗ and hence N may be considered as a σi∗ -overfield of K (i) a0 , . . . , ar ∗ . By Proposition 2.1.8 (parts (ii) and (iii) ), θ can be extended to an isomorphism θ˜ of K (i) a0 , . . . , ar ∗1 onto K (i) a1 , . . . , ar , b∗1 such that τ˜1 and θ˜ coincide on K (i) a0 , . . . , ar−1 ∗ . Using the arguments of the preceding paragraph and Theorem 5.1.8 we obtain that ˜ τ1 (x) = θψ(x) there exists a σ1 -M1 -isomorphism ψ of M2 onto N such that ψ˜

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375

for any x ∈ M1 . Therefore, the contraction of ψ to K (i) a0 , . . . , ar+1  gives the equivalence (in the sense of isomorphism) of the generic prolongations of R. It remains to prove statement (ii). Let R = (K (i) a0 , . . . , ar+1 , τ  ) be a generic prolongation of R which satisfies property P∗ with respect to a triple (G1 , G2 , τ1 ). Let M be the separable part of G1 over K (i) a0 , . . . , ar−1 ∗ . Then τ1 (M ) ⊆ G1 . Indeed, if x ∈ τ1 (M ), then x is separably algebraic over K (i) a1 , . . . , ar ∗ and hence over G1 . Since the extension G2 /G1 is primary, x ∈ G1 . Furthermore, if r = 0, then x is separably algebraic over K hence τ1 (M ) ⊆ M . Let M1 denote the separable part of G1 over K (i) a0 , . . . , ar ∗ if r = 0 and let M1 = M τ1 (M ) if r > 0. Then K (i) a0 , . . . , ar ∗ ⊆ M1 and M ⊆ M1 ⊆ G1 . Denoting the restriction of τ1 on M by τ2 , one can easily check that R satisfies property P∗ with respect to (M, M1 , τ2 ). Let M2 = M1 τ1 (M1 ). Then K (i) a0 , . . . , ar+1 ∗ ⊆ M2 ⊆ G2 and the σi -field M2 is inversive. Since the field M1 is algebraic over K (i) a0 , . . . , ar ∗ , the fields M1 and K (i) a0 , . . . , ar+1 ∗ are algebraically disjoint over K (i) a0 , . . . , ar ∗ . Furthermore, since R is a generic prolongation of R, Theorem 1.6.44(ii) implies that the fields τ1 (M ) and K (i) a1 , . . . , ar+1 ∗ are algebraically disjoint over K (i) a1 , . . . , ar ∗ and also M1 and K (i) a1 , . . . , ar+1 ∗ τ1 (M ) are algebraically disjoint over τ1 (M ). Using this fact, the algebraic disjointness of K (i) a1 , . . . , ar+1 ∗ M1  and τ1 (M1 ) over K (i) a1 , . . . , ar+1 ∗ τ1 (M ) (which follows from the fact that τ1 (M ) is algebraic over K (i) a0 , . . . , ar+1 ∗ and hence over the field K (i) a0 , . . . , ar+1 ∗ τ1 (M )), and Theorem 1.6.44(i), we obtain that the fields M1 and τ1 (M ) are algebraically disjoint over K (i) a0 , . . . , ar ∗ . This completes the proof of the theorem.  Definition 6.1.11 With the notation introduced at the beginning of this section, a difference kernel R = (K (i) a0 , . . . , ar , τ ) is said to satisfy the stepwise compatibility condition if the σi -field K (i) a0 , . . . , ar  satisfies the stepwise compatibility condition (see Definition 5.1.13). Theorem 6.1.12 If a difference kernel R satisfies the stepwise compatibility condition, it also satisfies property P∗ . PROOF. Suppose that a kernel R = (K (i) a0 , . . . , ar , τ ) satisfies the stepwise compatibility condition. Then the σi -field K (i) a0 , . . . , ar−1 ∗ also satisfies this condition. Furthermore, by Theorem 5.1.15 and Proposition 2.1.9(ii), there exists an algebraic closure L of K (i) a0 , . . . , ar−1 ∗ which has a structure of a σi∗ -overfield of K (i) a0 , . . . , ar−1 ∗ . Since any two σi -field extensions of the last field are compatible, we may assume that L and K (i) a0 , . . . , ar ∗ have a common σi -overfield M . Furthermore, by Theorem 5.1.9, τ has an extension to an isomorphism τ˜ of L into a σi -overfield N of M . If r = 0, let L1 = K (i) a0 ∗ L, τ (L). It follows from the definition of L that the fields and if r ≥ 1, let L1 = L˜ L and K (i) a0 , . . . , ar ∗ are algebraically disjoint over K (i) a0 , . . . , ar−1 ∗ and  that the field extension L1 /L is primary. Thus, R satisfies property P∗ .

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Note that the converse statement for the last theorem is not true. The following example, given in [10], introduces a kernel which satisfies L∗ and does not satisfy the stepwise compatibility condition. Example 6.1.13 Let F be the inversive difference field Q with the basic set σ = {α1 , α2 } described in Example 6.1.9 and let F (2) denote the field F treated as an ordinary difference field with the basic set σ2 = {α1 }. Let x be any element transcendental over F (x belongs to some overfield of F (2) ). Then α1 can be extended to an automorphism of F (x) that maps x onto x. This extension (also denoted by α1 ) defines a structure of σ2 -overfield of F (2) on F (x); we denote this σ2 -overfield by F (2) x. Let us consider the kernel R = (F (2) x, α2 ). Since the kernel in Example 6.1.9 does not satisfy P∗ , it follows from Theorem 6.1.12 that F (2) does not satisfy the stepwise compatibility condition. On the other hand the field extension F (x)/F is primary, hence R satisfies L∗ (it follows from Propositions 6.1.7 and 2.1.9(iii)).

6.2

Realizations of Difference Kernels over Partial Difference Fields

Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, let an index i ∈ {1, . . . , n} be fixed, and let K (i) denote the field K treated as a difference field with the basic set σi = σ \ {αi }. Furthermore, let R = (K (i) a0 , . . . , ar , τ ) be a difference kernel over K where a0 , . . . , ar are s-tuples (s ∈ N+ ). Let η = (η (1) , . . . , η (s) ) be an s-tuple in a σ-overfield L of K and let θ denote the extension of αi from K to L (usually we denote such an extension by the same symbol αi , but in this case it is convenient to use a special symbol). In what follows we set η0 = η and ηj = θj (η) (= (θj (η (1) ), . . . , θj (η (s) )) for j = 1, 2, . . . . Definition 6.2.1 With the above notation, we say that η is a realization of the kernel R in L over K if the set (η0 , . . . , ηr ) is a specialization over K (i) (k) (k) of (a0 , . . . , ar ) with aj → ηj (1 ≤ k ≤ s, 0 ≤ j ≤ r). We also say that θ is a realization of τ in L over K. If (η0 , . . . , ηr ) is a generic specialization of (a0 , . . . , ar ), η is said to be a regular realization of the kernel R. If there exists a sequence of kernels R0 , R1 , R2 , . . . such that R0 = R, Rj+1 is a generic prolongation of Rj , and η is a regular realization of Rj over K (j = 0, 1, . . . ), then η is called a principal realization of R. Definition 6.2.2 Two realizations η and ζ of a kernel R over a difference (σ-) field K are called equivalent if there is a σ-K-isomorphism φ : Kη → Kζ such that φ(η (k) ) = ζ (k) for k = 1, . . . , s. Remark 6.2.3 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, let ζ be an s-tuple with coordinates in some σ-overfield L of K, and let θ denote the extension of some αi (1 ≤ i ≤ n) to an injective endomorphism of L (in most cases this extension is denoted by the same symbol αi ). Furthermore,

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377

let K (i) denote the field K treated as a difference field with the basic set σi = σ \ {αi } and let θ denote the σi -isomorphism of K (i) ζ, θ(ζ), . . . , θs−1 (ζ) onto K (i) θ(ζ), θ2 (ζ), . . . , θs (ζ). Then it is easy to see that ζ is a regular realization of the kernel (K (i) ζ, θ(ζ), . . . , θs (ζ), θ ) in L over K. Theorem 6.2.4 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, let an index i ∈ {1, . . . , n} be fixed, and let K (i) denote the field K treated as a difference field with the basic set σi = σ \ {αi }. Furthermore, let R = (K (i) a0 , . . . , ar , τ ) be a difference kernel over K where a0 , . . . , ar are s-tuples (s ∈ N+ ). Then the following conditions are equivalent. (i) R satisfies P∗ . (ii) R has a principal realization. (iii) R has a regular realization. PROOF. Suppose that R satisfies P∗ . By Theorem 6.1.10, there exists a sequence of kernels Rj = (K (i) a0 , . . . , ar+j , τj ) (j = 0, 1 . . . ,) such that R0 = R and for every j ∈ N, Rj+1 is a generic prolongation of Rj which satisfies P∗ . ∞  Then the field L = K (i) a0 , . . . , ar+j , together with the union of the isoj=0

morphisms τj (j ≥ 0) as an extension of αi , becomes a σ-overfield of K in which a0 is a principal realization of R over K. Thus, (i) implies (ii). Since the implication (ii)⇒(iii) is obvious, it remains to show that (iii) implies (i). Let η be a regular realization of R and let L be the inversive closure of Kη. Let θ denote the translation of L which is a realization of τ and let Lθ denote the difference field obtained from L by deletion of the translation θ from the basic set of L (the basic set of Lθ will be denoted by σθ ). Clearly, the σθ -field Lθ is inversive and θ is a σθ -isomorphism of this field into itself. Let S denote the separable part of Lθ over Kη0 , . . . , ηr−1 ∗ (as before, we set ηj = θj (η) for j = 0, 1, . . . ,). If r = 0, let S1 denote the separable part of Lθ over Kη0 , . . . , ηr ∗ , and if r ≥ 1, let S1 denote the compositum Sθ(S) in Lθ . Then S and Kη0 , . . . , ηr ∗ are algebraically disjoint over Kη0 , . . . , ηr−1 ∗ , and the extension Lθ /S, and hence S1 /S, is primary. By Proposition 2.1.8 (parts (i) and (v)) and by the uniqueness of the separable part of a field extension, S is inversive, and for r = 0, S1 is inversive. If r ≥ 1, then parts (ii) and (iii) of Proposition 2.1.8 imply that S1 = Sθ(S) is inversive. If θ0 denotes the restriction of θ to a σθ -isomorphism of S into Lθ , then S1 = Sθ0 (S) if r ≥ 1, and θ0 (S) ⊆ S for r = 0. Since the specialization over K of (a0 , . . . , ar ) onto (η0 , . . . , ηr ) is generic, it extends uniquely to a σi -isomorphism of K (i) a0 , . . . , ar  onto Kη0 , . . . , ηr  which, in turn, extends to a σi -isomorphism of K (i) a0 , . . . , ar ∗ onto the field Kη0 , . . . , ηr ∗ and then to a difference isomorphism φ of a difference overfield N of K (i) a0 , . . . , ar ∗ onto S1 . If M is a difference subfield of N whose image under φ is S, then τ extends uniquely to a difference isomorphism τ˜ of M into N such that φ˜ τ = θ0 φ on M . It follows that the kernel R satisfies P∗ with respect to (M, N, τ˜). 

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Remark 6.2.5 The last part of the proof of the theorem and Theorem 6.1.10(ii) imply that every principal realization of a difference kernel is equivalent to one constructed as in the proof of the implication (i) ⇒ (ii) in Theorem 6.2.4. Corollary 6.2.6 With the notation of the theorem, suppose that a kernel R = (K (i) a0 , . . . , ar , τ ) satisfies L∗ . Then R has a principal realization over K and all principal realizations of R are equivalent. PROOF. Since R satisfies L∗ , this kernel also satisfies P∗ and hence has a principal realization. Let η and ζ be two principal realizations of R. Let R0 = R, R1 , R2 , . . . and R0 = R, R1 , R2 , . . . be the sequences of kernels associated with η and ζ, respectively, in the sense of the definition of principal realization (Definition 6.2.1). Since R satisfies L∗ , one can apply Theorem 6.1.10 and induction on j and obtain that the kernels Rj and Rj are equivalent (in the sense of isomorphism) for every j ∈ N. Since for every j ∈ N, η is a regular realization of Rj and ζ is a regular realization of Rj , there exists (by a composition of maps) a σi -K (i) -isomorphism φj of K (i) η0 , . . . , ηr+j  onto (k) (k) K (i) ζ0 , . . . , ζr+j  such that ηl → ζl (0 ≤ l ≤ r + j, 1 ≤ k ≤ s). Clearly, the  union of all φj defines a σ-isomorphism of Kη onto Kζ with η → ζ. We need the following two lemmas to prove an important result on principal realizations (see Theorem 6.2.9 below). Lemma 6.2.7 Let L and M be fields, let R1 and R2 be two subrings of L, and let R3 be a subring of R1 ∩ R2 . Let R = R1 [R2 ] (the smallest subring of L containing R1 and R2 ) and let φ be a homomorphism of R into M such that the contractions of φ to R1 and R2 are isomorphisms. Furthermore, suppose that the quotient fields in M of φ(R1 ) and φ(R2 ) are algebraically disjoint over the quotient field of φ(R3 ) in M . Then φ is an isomorphism of R into M . PROOF. For any subring A of L or M , let A denote the quotient field of A in L or M , respectively. Let R2 = R3 [S] and let B be a maximal algebraically independent over R1 subset of S. Since R3 ⊆ R1 , B is also algebraically independent over R3 , hence φ(B) is algebraically independent over φ(R3 ) and therefore over φ(R3 ). Since φ(R1 ) and φ(R2 ) are algebraically disjoint over φ(R3 ), φ(B) is algebraically independent over φ(R1 ). It follows that no nonzero element of R1 [B] is mapped to zero under φ, hence φ can be extended to a homomorphism φ of R1 (B)[S] onto φ(R1 )(φ(B))[φ(S)]. Since elements of S are algebraic over R1 [B], R1 (B)[S] is a field that can be identified with R1 (S) = R (see Theorem 1.6.4). It follows that φ is an isomorphism of R into M hence φ is an isomorphism as well.  Lemma 6.2.8 With the notation introduced in Theorem 6.2.4, let R1 = (K (i) a0 , . . . , ar , ar+1 , τ1 ) and R1 = (K (i) b0 , . . . , br , br+1 , τ1 ) (r ≥ 0) be kernels such that R1 is a generic prolongation of a kernel R = (K (i) a0 , . . . , ar , τ ) and there exists a specialization φ of (b0 , . . . , br , br+1 ) onto (a0 , . . . , ar , ar+1 ) over K (i) . Furthermore, suppose that φ contracts to a generic specialization φ1 of (b0 , . . . , br ) onto (a0 , . . . , ar ) over K (i) . Then the specialization φ is generic.

6.2. REALIZATIONS

379

PROOF. It is easy to see that φ1 extends to a σi -isomorphism of the σi -field K (i) b0 , . . . , br  onto K (i) a0 , . . . , ar . Because of the isomorphisms τ1 and τ1 , the contraction of φ to φ2 : (b1 , . . . , br+1 ) → (a1 , . . . , ar+1 ) is an isomorphism of K (i) {b1 , . . . , br+1 } onto K (i) {a1 , . . . , ar+1 }. By Proposition 2.1.7(v), φ extends uniquely to a σi -homomorphism of K (i) b0 , . . . , br+1 ∗ onto K (i) a0 , . . . , ar+1 ∗ , and φ contracts to a σi -isomorphism of K (i) b0 , . . . , br ∗ onto K (i) a0 , . . . , ar ∗ which is an extension of φ1 . Also, φ contracts to a σi -isomorphism of K (i) b1 , . . . , br+1 ∗ onto K (i) a1 , . . . , ar+1 ∗ which is an extension of φ2 , and φ maps K (i) b1 , . . . , br ∗ onto K (i) a1 , . . . , ar ∗ . Since R1 is a generic prolongation of R, Lemma 6.2.7 and the remark after the proof of Proposition 2.1.11 show that φ is an isomorphism. Thus, the specialization φ is generic.  Theorem 6.2.9 With the notation of Theorem 6.2.4, if η is a principal realization of a kernel R = (K (i) a0 , . . . , ar , τ ), then η is not a proper specialization over K of any other realization of R. PROOF. Suppose that η is a principal realization of R and ζ a realization of R such that there exists a specialization φ over K of ζ onto η with ζ (k) → η (k) (1 ≤ k ≤ s). Then φ is generic if for every j ∈ N, the contraction of φ to φj : K (i) {ζ0 , . . . , ζj } → K (i) {η0 , . . . , ηj } (ζl → ηl for l = 0, 1, . . . , j) is a σi isomorphism. Notice that the composition of the specialization over K (i) of (a0 , . . . , ar ) onto (ζ0 , . . . , ζr ) with the mapping φr agrees on K (i) {a0 , . . . , ar } with the assumed generic specialization over K (i) of (a0 , . . . , ar ) → (η0 , . . . , ηr ) . Therefore, φr is a σi -isomorphism. Suppose that φr+k is a σi -isomorphism for some k ≥ 0. By Lemma 6.2.8, φr+k+1 is also a σi -isomorphism, so one can apply induction on j and obtain that all φj (j ∈ N) are σi -isomorphisms. This completes the proof.  Exercise 6.2.10 Let K be an inversive difference field with a basic set σ and let η be an s-tuple with coordinates in some σ-overfield of K (s ≥ 1). Prove that there exists a kernel R over K such that η is the unique principal realization of R over K, and every realization of R over K is a specialization of η over K. [Hint: Generalize the arguments of the proof of Proposition 5.2.11.] Let K be a difference field with a basic set σ and let a = {a(i) | i ∈ I} be an indexing of elements in some σ-overfield of K. We say that this indexing is σ-algebraically independent over K if and only if all a(i) (i ∈ I) are distinct and form a σ-algebraically independent set over K (see the definition at the beginning of section 4.1). In what follows we keep our previous notation: for any difference kernel R = (K (i) a0 , . . . , ar , τ ) (K is an inversive difference field with a basic set σ = {α1 , . . . , αn } and an index i ∈ {1, . . . , n} is fixed) and for any subindexing (j ) (j ) (j ) (j ) b0 = (a0 1 , . . . , a0 k ) of a0 , the corresponding subindexing (al 1 , . . . , al k ) of al (1 ≤ l ≤ r) will be denoted by bl .

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Lemma 6.2.11 Let R = (K (i) a0 , . . . , ar , τ ) be a kernel (we use the above conventions about K and denote σ\{αi } by σi ) and let R1 = (K (i) a0 , . . . , ar+1 , τ1 ) be a generic prolongation of R. Furthermore, let b0 be a subindexing of a0 . (i) If br is σi -algebraically independent over K (i) a0 , . . . , ar−1 , then br+1
r+1 is σi -algebraically independent over K (i) a0 , . . . , ar  and the set j=0 bj is σi algebraically independent over K (i) . (ii) If b0 is σi -algebraically independent over K (i) a1 , . . . , ar , then b0 is σi
r+1 algebraically independent over K (i) a1 , . . . , ar+1  and the set j=0 bj is σi algebraically independent over K (i) . PROOF. (i) Because of the σi -isomorphism τ1 and our assumption on b0 , br+1 is σi -algebraically independent over K (i) a1 , . . . , ar  and hence also over K (i) a1 , . . . , ar ∗ . Since R1 is a generic prolongation of R, br+1 is σi algebraically independent over K (i) a0 , . . . , ar ∗ and hence over K (i) a0 , . . . , ar . Let us show that for each j, 0 ≤ j ≤ r + 1, bj is σi -algebraically independent over K (i) a0 , . . . , aj−1 . Indeed, our assumption and the result of the preceding paragraph show that this statement is true for j = r and j = r + 1. Suppose that j < r. Then br is σi -algebraically independent over K (i) ar−j , . . . , ar−1  and the composition of the corresponding restrictions of τ is a σi -isomorphism of K (i) a0 , . . . , aj  onto K (i) ar−j , . . . , ar . It follows that bj is σi -algebraically independent over K (i) a0 , . . . , aj−1 . Also, we obtain that for every j, 0 ≤ j ≤
r+1 r + 1, bj is σi -algebraically independent over K (i) b0 , . . . , bj−1 , hence j=0 bj is σi -algebraically independent over K (i) . (ii) Since b0 is σi -algebraically independent over K (i) a1 , . . . , ar , it is σi -algebraically independent over K (i) a1 , . . . , ar ∗ and hence over the field K (i) a1 , . . . , ar+1 ∗ (because the prolongation R1 of R is generic) and over K (i) a1 , . . . , ar+1 .
j Let j be an integer, 0 ≤ j ≤ r such that k=0 bk is σi -algebraically independent over K (i) (obviously, this is true for j = 0). Since the restriction of
j+1 τ1 to K (i) is an automorphism of K (i) , the set k=1 bk is also σi -algebraically shows that independent over K (i) . At the same time, the previous paragraph
r+1 b0 is σi -algebraically independent over K (i) b1 , . . . , bj+1 , hence j=0 bj is σi  algebraically independent over K (i) . Theorem 6.2.12 Let R = (K (i) a0 , . . . , ar , τ ) be a kernel (we use the notation and conventions of the preceding lemma), let η = (η (1) , . . . , η (s) ) be a principal realization of R, and let b0 be a subindexing of a0 . (i) If br is σi -algebraically independent over K (i) a0 , . . . , ar−1 , then the corresponding subindexing ζ of η is σ-algebraically independent over K. (ii) If b0 is σi -algebraically independent over K (i) a1 , . . . , ar , then the corresponding subindexing ζ of η is σ-algebraically independent over K. PROOF. Since η is a principal realization of R, for every j ∈ N there exists a kernel Rj = (K (i) a0 , . . . , ar+j , τj ) obtained from R by a sequence of generic

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381

prolongations and such that η is a regular realization of Rj over K. Using part
r+j (i) of Lemma 6.2.11 and induction on j we obtain that the set k=0 bk is σi
r+j algebraically independent over K (i) and hence k=0 τ˜k (ζ), where τ˜ denotes the realization of τ , is also σi -algebraically independent over K (i) . Since this is true for every j ∈ N, ζ is σ-algebraically independent over K. The proof of part (ii) is similar; we leave it to the reader as an exercise.  We conclude this section with an important result on principal realizations of ordinary difference kernels. Let K be an inversive ordinary difference field with a basic set σ = {α}. Let R = (K(a0 , . . . , ar ), τ ) be a difference kernel over K with a principal realization (1) (s) η = (η 1 , . . . , η s ) (ai is an indexing (ai , . . . , ai ) for i = 0, 1, . . . ). The following example, which is due to R. Cohn [41, Chapter 10, Section 8], shows that if R = (K(a0 , . . . , ar ), τ ) is a kernel which specializes to R, there may be no principal realization of R which specializes to η. At the same time, Theorem 6.2.14 below presents one case where such a specialization of a principal realization does exist. Example 6.2.13 Let K be an inversive ordinary difference field with a basic set σ = {α} and let a kernel R = (K(a, b), τ ) over K be defined as follows: a and b are 4-tuples whose coordinates a(i) and b(i) (1 ≤ i ≤ 4) are chosen in such a way that a(1) , b(1) , a(2) , b(2) , and a(3) form an algebraically independent set over K, b(3) = a(3) b(1) (a(2) )−1 , and a(4) = a(3) b(1) (a(1) )−1 . It is easy to see that the coordinates of a, as well as coordinates of b, are algebraically independent over K, so the kernel R is well-defined (with a = a0 and b = a1 , and if one uses the standard notation for kernels, τ maps a(i) to b(i) for i = 1, . . . , 4 and τ |K = α). The realizations of R are solutions of the system y1 (αy4 ) = y2 (αy3 ) = y3 (αy1 ) = y4 (αy2 ). (The corresponding σ-polynomials belong to the σ-polynomial ring K{y1 , y2 , y3 , y4 }.) Since trdegK K(a, b) = 5 and trdegK K(a) = 4, δR = 1. Furthermore, a(3) specializes to 0 over K(a(1) , a(2) , b(1) , b(2) ), and this specialization maps (a(4) , b(3) , b(4) ) onto zero. Thus, (a(1) , a(2) , 0, 0; b(1) , b(2) , 0, 0) is a specialization of (a, b). It follows that η = (η (1) , η (2) , 0, 0), with η (1) and η (2) σ-algebraically independent over K, is a realization of R. Since σ-trdegK Kη = 2 > δR, this realization is not principal (see Theorem 5.2.10). The following result is due to B. Lando [117]. Theorem 6.2.14 Let K be an inversive ordinary difference field with a basic set σ = {α}. Let R = (K(a0 , . . . , ar ), τ ) and R = (K(a0 , . . . , ar ), τ ) be two kernels (1) (s) (1) (s) over K (r ∈ N, ai = (ai , . . . , ai ), and ai = (ai , . . . , ai ) for i = 0, 1, . . . , r) such that there is a K-isomorphism of K(a0 , . . . , ar−1 onto K(a0 , . . . , ar−1 ) (in this case we write K(a0 , . . . , ar−1 ) ∼ =K K(a0 , . . . , ar−1 )). Furthermore, let (1) (s) η = (η i , . . . , η i ) be a principal realization of R. Then there exists a principal (1) (s) realization η = (ηi , . . . , ηi ) of R which specializes to η over K.

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PROOF. Using Remark 5.2.12 one can assume that r = 0 or r = 1. We shall give a proof with the assumption that r = 1 (the case r = 0 can be treated similarly). Since K(a0 ) ∼ =K K(a0 ), we may assume that a0 = a0 . Since (a1 ) specializes to (a1 ) over K(a0 ), δR ≥ δR. If δR = δR, then K(a0 , a1 ) ∼ =K K(a0 , a1 ), hence a principal realization of R specializes to η over K. Thus, it remains to consider the non-trivial case a0 = a0 , δR = 0 and δR = 1. Clearly, it is sufficient to show that for any m > 0, a kernel (K(a0 , . . . , am ), τ ) can be found with (K(a0 , . . . , ak+1 ), τ ) a generic prolongation of (K(a0 , . . . , ak ), τ ) (k = 1, . . . , m − 1 and the corresponding extensions of τ are denoted by the same letter τ ) such that (a0 , . . . , am ) specializes to (η, α(η), . . . , αm (η)). Indeed, if the statement of the theorem is false, then no principal realization of R specializes to η. Since the number of distinct principal realizations is finite (see Theorem 5.2.10(iii)) there exists an integer N > 0 such that for any principal realization η of R, (η, . . . , αN (η)) does not specialize to (η, . . . , αN (η)). Let L be the algebraic closure of Kη. By taking free joins and isomorphic kernels if necessary, we may assume that (η, α(η)) = (a0 , a1 ), that trdegL L(a1 ) = 1 and that (a0 , a1 ) specializes to (a0 , a1 ) over L (we write this as (a0 , a1 ) →L (a0 , a1 )). Then for any given m > 1, there exist s-tuples b2 , . . . , bm such that (i) K(a1 , b2 , . . . , bm ) ∼ =K K(a1 , α2 (η), . . . , αm (η)), (ii) (a0 , a1 , b2 , . . . , bm ) →L (a0 , a1 , α2 (η), . . . , αm (η)), (iii) trdegL L(a0 , a1 , b2 , . . . , bm ) = 1. (To see this, it is sufficient to take into account the result of Proposition 1.6.42 and the K-isomorphism K(a1 ) ∼ =K K(a1 ).) Now, in order to continue the proof we need the following fact. Lemma 6.2.15 Let L be an algebraically closed field and L(b) = L(b(1) , . . . , b(n) ) where the n-tuple b = (b(1) , . . . , b(n) ) of elements of an overfield of L satisfies (1)

(n)

the condition trdegL L(b) = 1. Furthermore, let b = (b , . . . , b ) be an n-tuple of elements of L. If b specializes to b over L, then there exists a parameter t ∈ L(b), transcendental over L, such that L[b] has a representation in the ∞  (i) cij tj (i = 1, . . . , n). power series ring L[[t]] with b(i) = b + j=1

PROOF. Clearly, L(b) is a field of algebraic functions of one variable over L in the sense of the definition in the last part of Section 1.6. Using the results of this part, we obtain a place p and a valuation ring A of L(b) over L such (i)

that L[b] ⊆ A and b(i) − b ∈ p for i = 1, . . . , n (see Proposition 1.6.70). By Proposition 1.6.69(i), A/p is algebraic over L. Moreover, since the field L is algebraically closed, A/p = L. Applying Proposition 1.6.71 (and taking into account the trivial fact that A/p = L is separable over L) we obtain that every ∞  cj tj with element c in the p-adic completion of L(b) has a representation c = j=r

r ∈ Z and cj ∈ L for all j. Also, if c ∈ A, then r ≥ 0; and if c ∈ p, then r ≥ 1.

6.2. REALIZATIONS

∞ 

383

Since L[b] ⊆ A, we have L[b] ⊆ L[[t]], and since b(i) − b

(i)

∈ p, b(i) − b

(i)

cij tj with b , cij ∈ L. This completes the proof of the lemma.

(i)

= 

j=1

COMPLETION OF THE PROOF OF THE THEOREM Applying the last lemma we obtain a parameter t ∈ L(a1 , b2 , . . . , bm ), transcendental over L, such that L[a1 , b2 , . . . , bm ] ⊆ L[[t]] with (i)

(i)

a1 = a1 +

∞ 

cij tj

(i = 1, . . . , n);

j=1 (i)

bk = αk (η (i) ) +

∞ 

dkij tj

(i = 1, . . . , n; k = 2, . . . , m).

(6.2.1)

j=1

The mapping τ : K(a0 ) → K(a1 ) of the kernel R may be extended to an isomorphism of K(a0 , a1 , α2 (η), . . . , αm−1 (η) onto K(a1 , b2 , . . . , bm ) obtained by the composition of the K-isomorphism from K(a0 , a1 , α2 (η), . . . , αm−1 (η)) to the field K(a1 , α2 (η), . . . , αm (η)) generated by τ and the K-isomorphism K(a1 , α2 (η), . . . , αm (η)) → K(a1 , b2 , . . . , bm ) in (i). This K-isomorphism, as well as its further extensions, will be denoted by the same letter τ (it cannot cause any confusions). At the next step, one can extend τ to a monomorphism of L to the algebraic closure L1 of the quotient field of L[[t]]. This monomorphism, in turn, can be extended to a monomorphism of L[[t]] into L1 [[t1 ]] where an element t1 is transcendental over L1 and τ (t) = t1 . Repeating this method of extension, for each k = 2, . . . , m − 1 we can extend τ to a monomorphism Lk−1 [[tk−1 ]] → Lk [[tk ]] with Lk−1 [[tk−1 ]] ⊆ Lk and tk transcendental over Lk (according to our convention, this monomorphism is also denoted by τ ). (i) Since a1 ∈ L[[t]] (i = 1, . . . , s), τ k (a0 ) = τ k−1 (a1 ) ∈ Lk−1 [[tk−1 ]] (2 ≤ k ≤ m). Let ak = τ k (a0 ). Then K[a0 , a1 , . . . , am−1 ] ⊆ Lm−2 [[tm−2 ]], and τ restricted to K[a0 , a1 , . . . , am−1 ] is an isomorphism onto K[a1 , . . . , am ]. Therefore, (K(a0 , a1 , . . . , am ), τ )) is a kernel which can be obtained from R by m − 1 prolongations. In what follows, let b denote the (m − 1)s-tuple (b2 , . . . , bm ). we are going to show that the kernel (K(a0 , a1 , . . . , am ), τ )) is obtained from R by generic prolongations. Indeed, since t ∈ L(a1 , b), tk = τ k (t) ∈ Lk (ak+1 , τ k (b)), 1 ≤ k ≤ m − 1. Also, the fact that tk is transcendental over Lk implies that trdegLk Lk (ak+1 , τ k (b) ) ≥ 1.

(6.2.2)

Applying (i), Proposition 1.6.31(iii), and the fact that δR = 0, we obtain that trdegK(a0 ,...,ak+1 ) K(a0 , . . . , ak+1 , τ k (b)) ≤ trdegK(ak+1 ) K(ak+1 , τ k (b)) = trdegK(a1 ) K(a1 , b) = trdegK(a1 ) K(a1 , α2 (η), . . . , αm (η)) = 0.

(6.2.3)

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384

Combining (6.2.2), (6.2.3) and the inequality in Proposition 1.6.31(iii), we obtain that trdegK(a0 ,...,ak ) K(a0 , . . . , ak+1 ) = trdegK(a0 ,...,ak ) K(a0 , . . . , ak+1 , τ k (b)) ≥ trdegLk Lk (ak+1 , τ k (b)) ≥ 1. At the same time, Proposition 1.6.31(iii) implies that trdegK(a0 ,...,ak ) K(a0 , . . . , ak+1 ) ≤ trdegK(ak ) K(ak , ak+1 ) = trdegK(a0 ,...,ak ) K(a0 , . . . , ak+1 , τ k (b)) = trdegK(a0 ) K(a0 , a1 ) = δR = 1. It follows that trdegK(a0 ,...,ak ) K(a0 , . . . , ak+1 ) = 1 for every k ≥ 0, therefore (K(a0 , . . . , am ), τ ) is obtained from R by generic prolongations. Let φk : Lk [[tk ]] → Lk be a Lk -homomorphism defined by φk (tk ) = 0, (i) (i) 1 ≤ k ≤ m−1. Using the series expansion (6.2.1) we obtain that a2 = τ (a1 ) = ∞ ∞ ∞ ∞     (i) (i) τ (a1 )+ (τ (cij ))tji = b2 + (τ (cij ))tji = α2 (η (i) )+ d2ij tj + (τ (cij ))tji . j=1

j=1

j=1

j=1

Therefore, (i)

ak = αk (η (i) ) +

∞ 

dkij tj +

j=1

∞ 

(τ (dk−1,ij ))tj1 + · · · +

j=1

∞ 

(τ k−1 (cij ))tjk−1

j=1

for k ≥ 2. Furthermore, K[a0 , . . . , ak+1 ] ⊆ Lk [[tk ]] and aq ∈ Lk−1 [[tk−1 ]], φk (aq ) = aq for q ≤ k. On the other hand, (i)

φk (ak+1 ) = φk (αk+1 (η (i) ) +

∞  j=1

+··· +

∞ 

dk+1,ij tj + · · · +

∞ 

τ k (cij ))tjk ) = αk+1 (η (i) )

j=1

(τ k−1 (dk+1,ij tj ))tjk−1 ∈ Lk−1 [[tk−1 ]].

j=1

Thus, φ = φ0 ◦φ1 · · · ◦φm−1 is a well-defined homomorphism on K[a0 , . . . , am ] with φ(ak ) = αk (η) (k = 0, . . . , m), hence (a0 , . . . , am ) →K (a0 , a1 , α2 (η), . . . ,  αm (η)). This completes the proof of the theorem. Corollary 6.2.16 Every regular realization of a kernel is the specialization of a principal realization. PROOF. Let R = (K(a0 , . . . , ar ), τ ) be a kernel of length r ≥ 0 over an ordinary difference field K with a basic set σ = {α}, and let η be a regular realization of R. By Theorem 5.2.10, σ-trdegK Kη ≤ δR. We proceed by induction on m = δR − σ-trdegK Kη. If m = 0, then Theorem 5.2.10(v) implies that η is a principal realization. Suppose that m > 0 and the result is true for any kernel R such that δR − σtrdegK Kη < m. For any k ≥ 0, let Rk denote the kernel with the field K(η, . . . , αr+k (η)). Then δR ≥ δR1 ≥ · · · ≥ δRk ≥ 0.

6.3. DIFFERENCE VALUATION RINGS

385

Let d be the minimal k ≥ 0 such that δRk = δRk+h for all h ≥ 0. Then η is a principal realization of Rd . Since m > 0, d > 0, and by the minimality of d, δRd−1 > δRd . It follows that Rd is not a generic prolongation of Rd−1 . Let R be the generic prolongation with the field K(η, . . . , αr+d−1 (η), ζr+d ) which specializes to K(η, . . . , αr+d−1 (η), αr+d (η)). By Theorem 6.2.14, there is a principal realization ζ of R which specializes to η. Then ζ is a regular realization of R. Furthermore, since the specialization ζ →K η is not generic, δR − σtrdegK Kζ < δR − σ-trdegK Kη = m. By the induction hypothesis, there is a principal realization θ of R which specializes to ζ. Therefore, θ →K ζ →K η. 

6.3

Difference Valuation Rings and Extensions of Difference Specializations

Definition 6.3.1 Let K be a difference field with a basic set σ = {α1 , . . . , αn }. A difference (or σ-) specialization of K is a σ-homomorphism φ of a σ-subring R of K onto a difference (σ-) domain Λ. If φ cannot be extended to a σhomomorphism of a larger σ-subring of K onto a domain which is a σ-overring of Λ, then φ is said to be a maximal difference (or σ-) specialization of K. It is easy to see that if φ is a maximal σ-specialization of a difference (σ-) field K, then the image Λ of the corresponding σ-subring R of K is a σ-field. Indeed, if φ(x) = 0 (x ∈ R), then one can define φ(x−1 ) as the element (φ(x))−1 in the quotient field of Λ. Since φ is maximal, one should have (φ(x))−1 ∈ Λ. Also, if φ is a maximal σ-specialization of a difference field K with a domain R ⊆ K, then M = Ker φ is a prime reflexive difference ideal of R which consists of the nonunits of R. Thus, R is a difference local ring with maximal ideal M . Definition 6.3.2 Let K be a difference field with a basic set σ and let φ be a maximal σ-specialization of K. Then the domain R of φ is called a maximal difference (or σ-) ring of K. If K is the quotient field of R, we say that R is a difference (or σ-) valuation ring of K, and φ is called a difference (or σ-) place of the σ-field K. If a difference field K is inversive, then every its maximal σ-ring is also inversive. It follows from the fact that every difference homomorphism of a difference subring R of K can be extended to a homomorphism of the inversive closure of R in K. Definition 6.3.3 A difference ring R with a basic set σ is called a local difference (or σ-) ring if the nonunits of R form a σ-ideal. This ideal is denoted by M (R). As we have mentioned after Proposition 2.1.10, for any difference ring A with a prime difference ideal P , the ring AP is a local difference ring if and only if P is reflexive. Therefore, the maximal ideal M (R) of a local difference ring R is reflexive (in this case R = RM (R) ).

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CHAPTER 6. DIFFERENCE KERNELS AND VALUATION RINGS

Definition 6.3.4 Let R be a local difference ring with a basic set σ and maximal R0 of R is a local σ-ring with a maximal ideal (σ ∗ -) ideal M (R). If a σ-subring  M (R0 )such that M (R) R0 = M (R0 ), then R is said to dominate R0 . In what follows, we present some results on extensions of difference specializations of difference fields which are natural generalizations of the corresponding statements proved in [118] for ordinary difference rings and fields. Proposition 6.3.5 Let K be a difference field with a basic set σ and R a local σ-subring of K. Then the following statements are equivalent. (i) R is a maximal σ-ring of K. (ii) R is maximal among local σ-subrings of K ordered by domination. (iii) If x ∈ K \ R, then the perfect σ-ideal {R{x}M (R)} generated by M (R) in R{x} coincides with R{x}. PROOF. (i)⇒ (ii). If R is a local σ-subring of K dominating R, then the natural σ-epimorphism R → R /M (R ) (M (R ) is the maximal ideal of the ring R ) would be an extension of the maximal σ-specialization of K with the domain R. Since the implications (ii)⇒ (i) and (iii)⇒ (ii) are obvious, it is sufficient to prove that (i) implies (iii). Let R be as in (ii) and x ∈ K \R. If the perfect σ-ideal {R{x}M (R)} of R{x} does not contain 1, it is contained in a reflexive prime ideal P of R{x}. Then one can extend the σ-specialization R → R/M (R) of the σ-field K to a natural σ-specialization R{x} → R{x}/P of K. This contradicts the fact that R is a maximal σ-ring of K.  Let K be a difference field with a basic set σ and R a difference valuation ring of K. Then the set U of units of R forms a subgroup of K  = K \{0} and one may define the natural homomorphism v : K  → K  /U . Let K  /U be denoted by Υ, with the operation written as addition. Then v is called a difference (or / Υ+ . σ-) valuation of K. Let Υ+ = v(M (R)\{0}); then for a ∈ Υ+ we have −a ∈ + For any a, b ∈ Υ we define a < b if b − a ∈ Υ . Then Υ becomes a partially ordered group which is not necessarily linearly ordered. Clearly, x ∈ R \ {0} if and only if v(x) ≥ 0, and x ∈ M (R) \ {0} if and only if v(x) > 0. The following example, due to R. Cohn, shows that there are difference valuation rings which are not valuation rings (and, thus, difference valuations which are not valuations). Example 6.3.6 Let us consider the field of complex numbers C as an ordinary difference field whose basic set σ consists of an identity automorphism α. Let a be transcendental over C and let C(a) be considered as a σ-overfield of C such that αa = −a (thus, Ca = C(a)). Let Ca{y} be the ring of σ-polynomials in one σ-indeterminate y over Ca and let A = y 2 − (αy)2 + ay 2 (αy)2 ∈ Ca{y}. Then A + αA = (y + α2 y)(y − α2 y)(1 + a(αy)2 ) and the variety M(A) of the difference polynomial A has two components, one satisfying y + α2 y, and the other satisfying y − α2 y. Let η be a generic zero of an irreducible component with α2 η + η = 0. Clearly, η is a solution of a σ-polynomial F ∈ Ca{y} if and

6.3. DIFFERENCE VALUATION RINGS

387

only if η is a solution of a first-order σ-polynomial F  obtained by substituting −y for α2 y. Since F  is a multiple of A, we obtain that η specializes to 0 over Ca. This specialization can be extended to a maximal one and, hence, there is a difference valuation ring R of Ca, η with η ∈ R. On the other hand, R is not a valuation ring, because the element ζ = α(η)η −1 is integral over R (it is / R. If φ is the a solution of the polynomial X 2 − (1 + a(αη)2 ) ∈ R[X]), but ζ ∈ maximal specialization and φ(ζ) is defined, then φ(ζ 2 ) = φ(1 + a(αη)2 ) = 1 and φ(ζα(ζ)) = 1. But ζα(ζ) = ((αη)η −1 )(α2 η)(αη)−1 = −1. Thus, ζ is not in the domain of φ. Theorem 6.3.7 Let R be a local difference subring of a difference field K with a basic set σ, M (R) the maximal ideal of R, and x ∈ K. Then the natural σhomomorphism φ : R → R/M (R) extends to one sending x to 0 if and only if 1∈ / [x] in R{x}. PROOF. If φ extends to a σ-homomorphism φ of R{x} such that φ (x) = 0, / [x]. Conversely, let N denote the difference ideal then [x] ⊆ Ker φ whence 1 ∈ of R{x} generated by M (R) and x, that is, N = R{x}M (R) + [x] = M (R) + [x]. Note that if φ cannot be extended to a σ-homomorphism of R{x} sending x to 0, then 1 ∈ {N }. (If 1 ∈ / {N }, then N is contained in a prime σ-ideal P of R{x}. Since P ∩ R = M (R), φ can be extended
to a σ-homomorphism ∞ R{x} → R{x}/P ). Recall (see Section 2.3) that {N } = k=0 Nk where N0 = N  and Nk = [Nk−1 ] for k = 1, 2, . . . . (As in Section 2.3, if S is a subset of a difference ring A with a basic set σ and Tσ is the free commutative semigroup generated by σ, then S  denote the set of all elements a ∈ A such that some product of the form τ1 (a)i1 . . . τm (a)im with τ1 , . . . , τm ∈ Tσ and i1 , . . . , im ∈ N belongs to S.) Let us show that if there is an element c ∈ {N } such that c = u + z for some u ∈ R \ M (R) and z ∈ [x], then 1 ∈ [x]. Then the proof will be completed, since 1 ∈ {N } and 1 can be expressed in this form (with z = 0). Considering the representation of {N } as a union of Nk , k = 0, 1, . . . we obtain that c ∈ [Nk ] for some k. Now we proceed by induction on k. If c ∈ [N0 ] = N = M (R) + [x], then c = u + z = m + w where m ∈ M (R), w ∈ [x]. Since u ∈ / M (R), u − m ∈ / M (R) hence (u − m)−1 ∈ R. Then 1 = (u − m)−1 (u − m) = (u − m)−1 (w − z) ∈ [x]. p  di gi where di ∈ R{x} and gi ∈ Let k ∈ N and c ∈ [Nk+1 ]. Then c = i=1

Nk+1 . For each i, di = ri + zi and gi = si + ti where ri , si ∈ R and zi , ti ∈ [x]. p p p    di gi = (ri + zi )(si + ti ) = v + w where v = ri si It follows that i=1

i=1

i=1

and w ∈ [x]. Thus, c = u + z = v + w. If v ∈ M (R), then, as above, (u − / M (R) for some i , 1 ≤ i ≤ p. Since v)−1 ∈ R and 1 ∈ [x]. Otherwise, si ∈ gi
= si
+ ti
∈ Nk+1 , there is a product of powers of transforms of gi
which lies in [Nk ]: π(gi
) = (si
+ ti
)l0 (τ1 (si
) + τ1 (ti
))l1 . . . (τq (si
) + τq (ti
))lq ∈ [Nk ] for some τ1 , . . . , τq ∈ Tσ . Setting u = sli0
(τ1 (si
))l1 . . . (τq (si
))lq we obtain that / M (R) and the ideal M (R) is prime π(gi
) = u + z  where z  ∈ [x]. Since si
∈ and reflexive, u ∈ R \ M (R). Applying the induction hypothesis we obtain that 1 ∈ [x]. 

388

CHAPTER 6. DIFFERENCE KERNELS AND VALUATION RINGS The following statement is a direct consequence of Theorem 6.3.7.

Corollary 6.3.8 Let R be a maximal difference ring of a difference field K. Let M (R) be the maximal ideal of R and let x ∈ K. Then x ∈ M (R) if and only if 1∈ / [x].  Let K be a difference field with a basic set σ and K{y} a ring of σpolynomials in one σ-indeterminate y over K. Let R be a σ-subring of K and let g be a σ-polynomial in R{y} with a constant term b ∈ R (by a constant term of a σ-polynomial we mean a term that does not contain any transform of a σ-indeterminate). Furthermore, let {g}R and {g}K denote perfect σ-ideals generated by g in the σ-rings R{y} and K{y}, respectively. Proposition 6.3.9 With the above notation, let the σ-ideal {g}K be prime, x a general zero of {g}K , and φ : R → Λ a difference specialization of K with φ(b) = 0. If {g}K R{y} = {g}R , then φ can be extended to R{x} with φ(x) = 0. PROOF. Let us show, first, that if f ∈ {g}R and c is the constant term ∞  [g]k , f ∈ [g]k of the σ-polynomial f , then φ(c) = 0. Indeed, since {g}R = k=0

for some k ≥ 0. We proceed by induction on k. If k = 0, then f =

p 

hi τi (g)

i=1

where hi ∈ R{y}, τi ∈ Tσ (1 ≤ i ≤ p). Comparison of the constant terms in the p  ci τi (b) (ci ∈ R is a constant term of hi , 1 ≤ i ≤ p) last equality yields c = i=1

whence φ(c) = 0. In order to make the induction step, it is sufficient to prove that if k ≥ 1 p  hi gi , where hi , gi ∈ R{y} and some product π(gi ) of powers of and f = i=1

transforms of gi lies in [g]k−1 (1 ≤ i ≤ p), then φ(c) = 0. Let ai denote the constant term of gi (1 ≤ i ≤ p). Then π(gi ) has constant term π(ai ) and by the induction hypothesis φ(π(ai )) = 0 whence φ(ai ) = 0. It follows that  p   φ(c) = φ ci ai = 0. i=1

In particular, we have shown that if f ∈ {g}R , then its constant term is not equal to 1. By extending φ in K, one  may assume that R is a localσ-subring of K and hence of K{x}, with {g}K  R{y} = {g}R . (If P = Ker φ and R is replaced by S = RP , then {g}K S{y} = {g}S .) By Theorem 6.3.7, if φ does not extend to a σ-homomorphism that sends x to 0, then 1 ∈ [x] in R{x}, that is, q  bi (x)τi (x) where τi ∈ Tσ and bi (x) ∈ R{x} is the result of substitution 1= i=1

of x for y in some σ-polynomial bi (y) ∈ R{y} (1 ≤ i ≤ q).

6.3. DIFFERENCE VALUATION RINGS It follows that f = 1 −

q 

bi (y)τi y ∈ {g}K

389 #

R{y}. Since the constant term

i=1

of f is 1, f ∈ / {g}R . Therefore, if the specialization φ cannot be extended to  R{x} with φ(x) = 0, then {g}K R{y} = {g}R . Proposition 6.3.10 Let K be a difference field with a basic set σ and let R be a maximal difference ring of K. Then (i) If S is another maximal difference ring of K such that R ⊆ S, then M (S) ⊆ M (R). (Therefore, in this case every ideal of S is an ideal of R.) (ii) Let P be a prime σ ∗ -ideal of R. Then there is a maximal difference ring R1 of K such that R ⊆ R1 and M (R1 ) = P . (iii) The prime reflexive σ-ideals of R are linearly ordered by inclusion. (iv) Let A be a maximal difference ring of K with a specialization φ : A → Λ, and let R ⊆ A. Then φ(R) is a maximal difference ring of Λ. PROOF. (i). By Corollary 6.3.8, if x ∈ M (S), then 1 ∈ / [x]S (where [x]S denotes the σ-ideal generated by x in S{x}). Since R ⊆ S, one also has 1 ∈ / [x]R whence x ∈ M (R). (ii). Let R1 be a maximal local difference ringof K dominating By (i),  RP .  ) ⊆ M (R). Therefore, M (R1 ) = M (R1 ) R = (M (R1 ) RP ) R = M (R1 P RP R = P . (iii). Let P be a prime σ-ideal of R. If x ∈ R\P and a ∈ P , then ax−1 ∈ P . (It follows from the fact that x is a unit in any maximal σ-ring A of K dominating RP and so ax−1 ∈ M (A); but by part (ii), M (A) = P .) Now, let P and Q be two reflexive prime σ-ideals of R. Suppose that Q  P , and let b ∈ P, c ∈ Q. As we have seen, bc−1 ∈ P hence b ∈ cP ⊆ Q. Since this is true for every b ∈ P , P ⊆ Q. (iv). Let ψ : R → Ω be a maximal specialization of K with domain R. Since Ker φ = M (A) ⊆ M (R) = Ker ψ, one can define the σ-homomorphism θ : φ(R) → Ω by θ(φ(r)) = ψ(r), r ∈ R. Clearly, Ker θ = φ(M (R)). Let x ∈ Λ. If x ∈ / φ(R), then there exists z ∈ A \ R such that φ(z) = x. Since ψ is maximal, 1 ∈ {R{z}M (R)} and thus 1 ∈ {φ(R){x}Ker θ}. Therefore, θ cannot be extended to x.  Theorem 6.3.11 Let K be a difference field with a basic set σ and R0 a σsubring of K with prime reflexive σ-ideals P and Q suchthat P ⊆ Q. Let S be there a proper maximal σ-ring of K with R0 ⊆ S and M (S) R0 = P . Then  exists a proper maximal σ-ring R of K such that R0 ⊆ R and M (R) R0 = Q. Furthermore, if S is a σ-valuation ring of K then R is also. PROOF. Let L = S/M (S) and let φ : S → L be the canonical σepimorphism. Then φ(R0 ) ⊆ L and the inclusion P ⊆ Q implies that the σ-homomorphism θ0 : φ(R0 ) → R0 /Q (φ(x) → x + Q for every x ∈ R0 ) is well-defined. Let us extend θ0 to a maximal σ-specialization θ of L. Let Rθ be the domain of θ, let φ = θ ◦ φ and R = φ−1 (M (Rθ )). Then R is a local

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CHAPTER 6. DIFFERENCE KERNELS AND VALUATION RINGS

−1 σ-subring of  K with the maximal σ-ideal φ (M (Rθ )). Clearly, R0 ⊆ R ⊆ S and M (R) R0 = Q. It is easy to check that R is a maximal σ-ring of K. The fact that the quotient field of R is K follows from the fact that M (S) ⊆ R. 

Corollary 6.3.12 Let K be a difference field with a basic set σ and R0 a local σ-subring of K. Let L be a σ-overfield of K and S a proper maximal σ-ring (valuation ring) of L containing R0 . Then there exists a proper maximal σ-ring (valuation ring) R of L dominating R0 .  PROOF. Clearly, M (S) R0 is a prime reflexive σ-ideal of R0 which is  contained in M (R0 ). It remains to apply Theorem 6.3.11. The existence of S in the corollary is equivalent to the condition that L have a subring S0 , S0 ⊆ R0 , which contains a proper nonzero prime σ ∗ -ideal. This condition does not always hold. For example, if Q is treated as an ordinary difference (σ-) field with the identity translation α and Q{b} is a σ-overring of Q such that b is transcendental over Q and α(b) = b, then the σ-specialization b → 1 does not extend to a σ-place Qa where a2 = b and α(a) = −a (see Example 5.3.4). However, the condition does hold in the situations of the Theorem 6.3.14 below. To prove this theorem we need the following statement that can be proved by using the arguments of the proof of Theorem 5.3.2 and applying Proposition 1.6.64. We leave the details to the reader as an exercise. Lemma 6.3.13 Let R be an ordinary difference integral domain with quotient field K. If L = Ka1 , . . . , am  is a difference field extension of K such that the field extension L/K is primary, then there exists a nonzero element u ∈ R such that any specialization φ of R with φ(u) = 0 can be extended to the ring  R{a1 , . . . , am }. Theorem 6.3.14 Let R0 be a local ordinary difference ring with a basic set σ = {α} and K the difference quotient field of R0 . (i) Suppose that R0 does not contain minimal nonzero prime reflexive σideals. If L is a primary finitely generated σ-field extension of K, then there exists a difference valuation ring R of L dominating R0 . (ii) Let b be an element from some σ-overfield of K which is σ-algebraically independent over K. If N = Kb, a1 , . . . , as  is a primary σ-field extension of Kζ, then there exists a difference valuation ring of N dominating R0 . PROOF. (i). Let x1 , . . . , xm be elements of L such that L = Kx1 , . . . , xm . Furthermore, let u be an element of R0 with the following property: for every u, there is a prime σ-ideal P  in the prime σ-ideal P of R0 not containing   R0 = P . (The existence of such an element ring R0 {x1 , . . . , xm } such that P follows from the preceding lemma.) By the hypothesis of the theorem, the intersection of all nonzero prime reflexive σ-ideals of R0 is (0). Therefore, one can choose a prime reflexive σ-ideal Q of R0 not containing u and hence obtain a prime reflexive σ-ideal Q of R0 {x1 , . . . , xm } such that Q ∩ R0 = Q.

6.3. DIFFERENCE VALUATION RINGS

391

 Let S be a σ-valuation ring of L such that M (S) R0 {x1 , . . . , xm } = Q . Applying Corollary 6.3.12 we obtain that there exists a σ-valuation ring R of L contained in S and dominating R0 . (ii). It is easy to see that the ideals Pk = {b − αk (b)} (k = 1, 2, . . . ) form a descending chain of prime reflexive σ-ideals in R0 {b}. Therefore, if / Pk for some k. Now the proof can be completed by u ∈ R0 {b}, u = 0, then u ∈ the arguments of the proof of part (i) (with the use of Lemma 6.3.13). 

Chapter 7

Systems of Algebraic Difference Equations 7.1

Solutions of Ordinary Difference Polynomials

In this section we start the discussion of varieties of ordinary difference polynomials. In what follows we use the terminology introduced for the ordinary case in Section 4.5. The whole section, as well as Sections 7.2 and 7.4 - 7.6, is based on the results of R. Cohn. Some of them were obtained in [28], but the main part of the theory first appeared in [41]. Proposition 7.1.1 Let K be an ordinary difference field with a basic set σ = {α} and let φ be a specialization of an s-tuple a = (a(1) , . . . , a(s) ) over K. ˜ denote the set {a(i1 ) , . . . , a(iq ) }, and Let {i1 , . . . , iq } be a subset of {1, . . . , s}, a (i1 ) (iq ) φ˜ a = {φa , . . . , φa }. Then (i) If φ˜ a is σ-algebraically independent over K, so is a ˜. Thus, σ-trdegK Kφa(1) , . . . , φa(s)  ≤ σ-trdegK Ka(1) , . . . , a(s) . (ii) If φ˜ a is a transcendence basis of {φa(1) , . . . , φa(s) } over K, then ord (1) a ≤ ord Ka(1) , . . . , a(s) /K˜ a, Eord Kφa(1) , . . . , Kφa , . . . , φa(s) /Kφ˜ (s) (1) (s) a ≤ Eord Ka , . . . , a /K˜ a, and the equality occurs if and φa /Kφ˜ only if the specialization φ is generic. PROOF. Part (i) follows from Proposition 5.3.1. If B ⊆ Ka(1) , . . . , a(s)  and a(B), then every element every element of Ka(1) , . . . , a(s)  is algebraic over K˜ a(φ(B)). Now the first statement of Kφa(1) , . . . , φa(s)  is algebraic over Kφ˜ of (ii) can be derived from Theorem 1.6.30. The last part of (ii) can be obtained similarly; we leave the details to the reader as an exercise.  Proposition 7.1.2 Let K be an ordinary difference field with a basic set σ = {α}, K{y1 , . . . , ys } the ring of σ-polynomials in s σ-indeterminates 393

394

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

y1 , . . . , ys over K, and η = (η1 , . . . , ηs ) a solution of a nonzero σ-polynomial A ∈ K{y1 , . . . , ys } (coordinates of η lie in some σ-overfield of K). Then either (I) σ-trdegK Kη < s − 1; or (II) σ-trdegK Kη = s − 1 and for every j ∈ {1, . . . , s} such that the elements η1 , . . . , ηj−1 , ηj+1 , . . . , ηs , are σ-algebraically independent over K, the following conditions hold: (a) the σ-polynomial A contains yj , (b) ord Kη/Kη1 , . . . , ηj−1 , ηj+1 , . . . , ηs  ≤ ordyj A, and (c) Eord Kη/Kη1 , . . . , ηj−1 , ηj+1 , . . . , ηs  ≤ Eordyj A. PROOF. Since A(η) = 0, the elements η1 , . . . , ηs are σ-algebraically dependent over K. Therefore, σ-trdegK Kη1 , . . . , ηs  ≤ s − 1. Suppose that σ-trdegK Kη1 , . . . , ηs  = s − 1, and for some j, 1 ≤ j ≤ s, the set {ηi | 1 ≤ i ≤ s, i = j} is σ-algebraically independent over K. Then the (s − 1)-tuple η  = (η1 , . . . , ηj−1 , ηj+1 , . . . , ηs ) annuls no σ-polynomial in K{y1 , . . . , yj−1 , yj+1 , . . . , ys }, hence A should contain some transform of yj . Let A be the σ-polynomial in Kη1 , . . . , ηj−1 , ηj+1 , . . . , ηs {yj } obtained from A by replacing every transform τ yi with i = j (τ ∈ Tσ ) by τ ηi . Then ordyj A = ordyj A and Eordyj A = Eordyj A . Since ηj is a solution of A , Corollaries 4.1.18 and 4.1.20 imply conditions (b) and (c).  Let K be an ordinary difference field with a basic set σ, K{y1 , . . . , ys } the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and M a nonempty irreducible variety over K{y1 , . . . , ys }. If (η1 , . . . , ηs ) is a generic zero of M, then σ-trdegK Kη1 , . . . , ηs , ord Kη1 , . . . , ηs /K, Eord Kη1 , . . . , ηs /K, and ld(Kη1 , . . . , ηs /K) are called the dimension, order, effective order, and limit degree of the variety M, respectively. They are denoted by dim M, ord M, Eord M, and ld M, respectively. If M = ∅, we set dim M = ord M = Eord M = −1 and ld M = 0. Obviously, if dim M > 0, then ord M = ∞. If P is a prime inversive difference ideal of K{y1 , . . . , ys } then the dimension, order, effective order, and limit degree of P (they are denoted by dim P , ord P , Eord P , and ld P , respectively) are defined as the corresponding values of the variety M(P ). (Thus, these concepts are determined as above through the σfield extension Kη1 , . . . , ηs /K where (η1 , . . . , ηs ) is a generic zero of P .) Let M be a nonempty irreducible variety over K{y1 , . . . , ys }, (η1 , . . . , ηs ) a generic zero of M, and {yi1 , . . . , yiq } is a subset of the set of σ-indeterminates {y1 , . . . , ys }. Then the dimension, order, effective order, and limit degree of M relative to yi1 , . . . , yiq are defined as σ-trdegK ηi1 ,...,ηiq  Kη1 , . . . , ηs , ord Kη1 , . . . , ηs /Kηi1 , . . . , ηiq , Eord Kη1 , . . . , ηs /Kηi1 , . . . , ηiq , and ld(Kη1 , . . . , ηs /Kηi1 , . . . , ηiq ), respectively. These characteristics of M are denoted by dim (yi1 , . . . , yiq )M, ord (yi1 , . . . , yiq )M, Eord (yi1 , . . . , yiq )M, and ld (yi1 , . . . , yiq )M, respectively. If P is a prime inversive difference ideal of K{y1 , . . . , ys } then the dimension, order, effective order, and limit degree of P relative to yi1 , . . . , yiq

7.1. SOLUTIONS OF ORDINARY DIFFERENCE POLYNOMIALS

395

are defined as the corresponding characteristics of M(P ) (the notation is the same: dim (yi1 , . . . , yiq )P, ord (yi1 , . . . , yiq )P, Eord (yi1 , . . . , yiq )(P ), and ld (yi1 , . . . , yiq )(P ), respectively). Definition 7.1.3 With the above notation, a subset {yi1 , . . . , yiq } of {y1 , . . . , ys } is called a set of parameters of P (or M(P )) if P contains no nonzero σpolynomial in F {yi1 , . . . , yiq }. A set of parameters of P which is not a proper subset of any set of parameters of P is called complete. Clearly, any reflexive prime σ-ideal P of K{y1 , . . . , ys } has at least one complete set of parameters, and every set of parameters can be extended to a complete one. Remark 7.1.4 If {yi1 , . . . , yiq } is a complete set of parameters of P , r = ord (yi1 , . . . , yiq )P , and η = (η1 , . . . , ηs ) is a generic zero of P , then there is an r-element transcendence basis B of Kη1 , . . . , ηs  over Kηi1 , . . . , ηiq  whose elements lie in the set {τ ηj | τ ∈ Tσ , 1 ≤ j ≤ s, j = iν for ν = 1, . . . , q} (the existence of such a transcendence basic is stated by Theorem 1.6.30(ii)). Let B = {τ1 ηj1 , . . . , τr ηjr } (τ1 , . . . , τr ∈ Tσ , 1 ≤ j1 ≤ · · · ≤ jr ≤ s) and let zk = τk yjk (1 ≤ k ≤ r). Then P ∩ K{yi1 , . . . , yiq }[z1 . . . . , zr ] = (0) .

(7.1.1)

Moreover, it is easy to see that ord (yi1 , . . . , yiq )P is the maximum integer r such that there exists an r-element subset {z1 , . . . , zr } of the set {τ yj | τ ∈ Tσ , 1 ≤ j ≤ s, j = iν for ν = 1, . . . , q}

(7.1.2)

with condition (7.1.1). Remark 7.1.5 With the above notation, let R = K{y1 , . . . , ys } and let {yi1 , . . . , yiq } be a complete set of parameters of a prime σ ∗ -ideal P of R. Let u1 = yj1 , . . . , us−q = yjs−q (1 ≤ j1 < · · · < js−q ≤ s) denote the σindeterminates of the set {y1 , . . . , ys }\{yi1 , . . . , yiq } and let R = Kyi1 , . . . , yiq  {u1 , . . . , us−q }. Furthermore, let P  = P R . Then it is easy to check that P  is a prime σ-ideal of R and if (η1 , . . . , ηs ) is a generic zero of P over K, then (ηq+1 , . . . , ηs ) is a generic zero of P  over Kyi1 , . . . , yiq  and ord (yi1 , . . . , yiq )P = ord (u1 , . . . , us−q )P  = ord P  . Proposition 7.1.6 With the above notation, a set of parameters of a reflexive prime difference ideal P of K{y1 , . . . , ys } is complete if and only if it contains dim P elements. PROOF. Let η = (η1 , . . . , ηs ) be a generic zero of the σ ∗ -ideal P . Then {yi1 , . . . , yiq } (1 ≤ i1 < · · · < iq ≤ s) is a set of parameters of P if and only if the set {ηi1 , . . . , ηiq } is σ-algebraically independent over K. Furthermore, it is easy to see that such a set of parameters is complete if and only if one has the  equalities q = σ-trdegK Kη1 , . . . , ηs  = dim P .

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Proposition 7.1.7 Let M1 and M2 be two irreducible varieties over the ring of σ-polynomials K{y1 , . . . , ys } (we use the above notation and conventions) such that M1 ⊆ M2 . Then (i) dim M1 ≤ dim M2 . (ii) If {yi1 , . . . , yiq } is a complete set of parameters of Φ(M1 ) (we use the notation of Section 2.6), then ord (yi1 , . . . , yiq )M1 ≤ ord (yi1 , . . . , yiq )M2 , and the equality occurs if and only if M1 = M2 . Furthermore, Eord (yi1 , . . . , yiq )M1 ≤ Eord (yi1 , . . . , yiq )M2 , and the equality occurs if and only if M1 = M2 . PROOF. Let Pi = Φ(Mi ) (i = 1, 2). Then P1 and P2 are reflexive prime difference ideals of K{y1 , . . . , ys } and P1 ⊇ P2 . Therefore, every complete set of parameters of P2 is a set of parameters of P1 . Applying Proposition 7.1.6 we immediately obtain the inequality in part (i). Let us prove the first part of (ii), the inequality and equality of orders. Let {yi1 , . . . , yiq } be a complete set of parameters of P1 (and therefore, a set of parameters of P2 ). Without loss of generality we may assume that {yi1 , . . . , yiq } is also a complete set of parameters of P2 (otherwise, ord (yi1 , . . . , yiq )M2 = ∞ and the statement is obvious). Let d = ord (yi1 , . . . , yiq )P1 and let z1 , . . . , zd be elements of the set (7.1.2) such that P1 ∩ K{yi1 , . . . , yiq }[z1 . . . . , zd ] = (0). Then P2 ∩ K{yi1 , . . . , yiq }[z1 . . . . , zd ] = (0), and the description of ord (yi1 , . . . , yiq )P given in Remark 7.1.4 shows that ord (yi1 , . . . , yiq )P1 ≤ ord (yi1 , . . . , yiq )P2 . Now, in order to complete the proof of the first part of (ii), we should show that if r = ord (yi1 , . . . , yiq )P2 and P1  P2 , then ord (yi1 , . . . , yiq )P1 < r. But this inequality is an immediate consequence of Remark 7.1.5 and Proposition 4.2.22 (with the notation of Remark 7.1.5, ord (u1 , . . . , us−q )P  is the σ-dimension polynomial of the ideal P  , see Definition 4.2.21). We leave the proof of the statement about effective orders to the reader as an exercise (one can use an analog of Remark 7.1.5 and the second part of Proposition 4.2.22).  Let K be an ordinary difference field with a basic set σ = {α} and R = K{y1 , . . . , ys } the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K. Let A ∈ R be an irreducible σ-polynomial, that is, A ∈ / K and A is irreducible as a polynomial in indeterminates αj yi (1 ≤ i ≤ s; j = 0, 1, . . . ). As before, M(A) will denote the variety of the σ-polynomial A, that is, a variety of the perfect σ-ideal {A}. Definition 7.1.8 With the above notation, an irreducible component M of the variety M(A) is called a principal component of this variety if whenever A contains a transform of yi (1 ≤ i ≤ s), the family {y1 , . . . , yi−1 , yi+1 , . . . , ys } is a complete set of parameters of M and Eord (y1 , . . . , yi−1 , yi+1 , . . . , ys )M is the effective order of A in yi . (This implies that dim M = s − 1.) Irreducible components of M which are not principal are called singular. Proposition 7.1.9 With the above notation, let A be a nonzero irreducible σpolynomial in K{y1 , . . . , ys } and η = (η1 , . . . , ηs ) a solution of A. Furthermore,

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397

suppose that σ-trdegK Kη1 , . . . , ηs  = s − 1, and for each j (1 ≤ j ≤ s) such that A contains a transform of yj , Eord Kη1 , . . . , ηs /Kη1 , . . . , ηj−1 , ηj+1 , . . . , ηs  = Eordyj A.

(7.1.3)

Then η is a generic zero of a principal component of M(A). PROOF. Clearly, η is contained in some irreducible component M of M(A). Let ζ be a generic zero of M. Then ζ specializes to η. By Propositions 7.1.1 and 7.1.2, σ-trdegK Kζ = s − 1. Suppose that some transform of yj (1 ≤ j ≤ s) appears in A. Then equality (7.1.3) shows that ηj is σ-algebraic over Kη1 , . . . , ηj−1 , ηj+1 , . . . , ηs . Since σ-trdegK Kη1 , . . . , ηs  = s − 1, the elements η1 , . . . , ηj−1 , ηj+1 , . . . , ηs form a σ-transcendence basis of Kη1 , . . . , ηs  over K. Applying Propositions 7.1.1 and 7.1.2 once again we obtain that Eord Kζ/Kζ1 , . . . , ζj−1 , ζj+1 , . . . , ζs  = Eordyj A. Since {y1 , . . . , yj−1 , yj+1 , . . . , ys } is a complete set of parameters of M, this variety is a principal component of M(A) (see Definition 7.1.8). Also, statement (ii) of Proposition 7.1.1 implies that the specialization of ζ to η is generic. This completes the proof.  Remark 7.1.10 Let K be an ordinary difference field with a basic set σ = {α} and let K{y1 , . . . , ys } be an algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys over K. Then the set of prime reflexive σ-ideals of K{y1 , . . . , ys } of dimension 0 is big enough in the following sense: if J is a perfect σ-ideal in K{y1 , . . . , ys } and A ∈ K{y1 , . . . , ys } \ J, then there exists a reflexive σ-ideal / P . Indeed, it follows P of K{y1 , . . . , ys } such that J ⊆ P , dim P = 0, and A ∈ from Theorem 5.4.28 (see also Lemma 5.4.31) that the ideal J has a solution η = (η1 , . . . , ηs ) which does not annul A. Then the prime σ ∗ -ideal P with generic zero η satisfies the required conditions. As a consequence we obtain that every proper perfect σ-ideal J of the algebra K{y1 , . . . , ys } is the intersection of all prime σ ∗ -ideals of K{y1 , . . . , ys } of dimension 0 which contain J. We conclude this section with a result which a strengthened version of Theorem 2.6.5 (the “difference Nullstellensatz”), see Theorem 1.7.13 below. Let K be an ordinary difference field with a basic set σ = {α}, K{y1 , . . . , ys } the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, M a non-empty irreducible variety over K{y1 , . . . , ys }, and N a variety over K{y1 , . . . , ys } which does not contain M. Proposition 7.1.11 With the above notation, the variety M contains a solution which is σ-algebraic over K and does not belong to N . PROOF. By Theorem 2.6.5, there is a σ-polynomial A ∈ Φ(N ) \ Φ(M) (we use the notation of Section 2.6). Let η = (η1 , . . . , ηs ) be a generic zero of M and let K  be the algebraic closure of K in Kη. Furthermore, let L be a completely aperiodic σ-field extension of K  such that the extension L/K  is

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σ-algebraic. (To construct L, one can consider the ring of σ-polynomials K  {z} in one σ-indeterminate z over K  and adjoin to K  a generic zero of the variety of the σ-polynomial αz − z − 1.) Applying Theorem 5.1.6 we obtain that the σfield extensions L/K and Kη/K are compatible. Let ζ = {ηi1 , . . . , ηik } be a σ-transcendence basis of η over K, let λ = {η1 , . . . , ηs } \ ζ, and let µ be the element of K{η} obtained by substituting ηj for yj in A (1 ≤ j ≤ s). Since A ∈ / Φ(M), µ = 0. By Theorem 5.4.28 there exists a nonzero σpolynomial B ∈ K{yi1 , . . . , yik } such that if η¯ is an indexing of k elements of a σ-overfield of K, B(¯ η ) = 0, and K¯ η /K is compatible with Kζ, λ/K, then ¯λ ¯ of ζ, λ such that λ ¯ is σ-algebraic over Kζ, ¯ there exists a σ-specialization ζ, ¯ ¯ and the σ-specialization of µ is not 0. Thus, ζ, λ lies in M, but not in N , since this s-tuple is not a solution of B. Furthermore, the fact that the extensions L/K and Kη/K are compatible and Proposition 4.5.3 imply that ζ¯ may be ¯λ ¯ is chosen as a set of elements of L. Then ζ¯ is σ-algebraic over K, hence ζ, σ-algebraic over K as well.  Using the previous notation, let us denote by Ma the set of all solutions of a difference variety M over K{y1 , . . . , ys } which are σ-algebraic over K. Then Proposition 7.1.11 immediately implies the following statement. Corollary 7.1.12 If M and N are two difference varieties over K{y1 , . . . , ys }  such that Ma = Na , then M = N . Theorem 7.1.13 Let K be an ordinary difference field with a basic set σ = {α}, K{y1 , . . . , ys } the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and Φ ⊆ K{y1 , . . . , ys }. Then a σ-polynomial A ∈ K{y1 , . . . , ys } belongs to {Φ} if and only if A is annulled by every solution of Φ which is σ-algebraic over K. PROOF. Since any σ-polynomial in {Φ} is annulled by any solution of Φ, one ∈ {Φ}. just needs to show that if A(η) = 0 for every η ∈ M(Φ)a , then A
Suppose that A ∈ / Φ. Then Theorem 2.6.5 implies that M(Φ) =  M(Φ {A}).
Applying Proposition 7.1.11 we obtain that M(Φ)a  M(Φ {A})a . Therefore, there is a solution of Φ which is σ-algebraic over K and does not annul A. This contradiction completes proof.  Let K be an ordinary difference field with a basic set α = {α}, L a σoverfield of K, and K{y1 , . . . , ys } and L{y1 , . . . , ys } the rings of σ-polynomials in σ-indeterminates y1 , . . . , ys over K and L, respectively. In what follows we shall denote these rings by K{y} and L{y}, respectively. Furthermore, for any set S ⊆ K{y}, [S] and {S} will denote, respectively, the difference (σ-)ideal and the perfect σ-ideal of K{y} generated by S, while [S]L and {S}L will denote, respectively, the σ-ideal and the perfect σ-ideal of L{y} generated by the set S. The following results, describing some relationships between characteristics of a prime σ ∗ -ideal P of K{y} and the essential prime divisors of {P }L , are due to R. Cohn. Theorem 7.1.14 With the above notation, let P be a prime σ ∗ -ideal of K{y}, let P1 , . . . , Pk be the essential prime divisors of {P }L , and let η = (η1 , . . . , ηs ) be a generic zero of P . Then

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399

(i) dim Pi ≤ dim P for i = 1, . . . , n. (ii) If Σ = {yj1 , . . . , yjq } is a complete set of parameters of some Pi (1 ≤ i ≤ k), then Σ is a (not necessarily complete) set of parameters of P and Eord (Σ)Pi ≤ Eord (Σ)P . Furthermore, if the σ-field extensions L/K and Kη/K are incompatible, then the last inequality is strict. (iii) If L/K and Kη/K are compatible, then there exists i, 1 ≤ i ≤ k, with the following properties: (a) dim Pi = dim P . (b) If Σ is a complete set of parameters of P , then Σ is a complete set of parameters of Pi and Eord (Σ)Pi = Eord (Σ)P . (iv) If L/K and Kη/K are compatible  and some divisor Pj (1 ≤ j ≤ k) has properties (a) and (b) of (iii), then Pj K{y} = P . PROOF. Let Pi be a prime divisor of P and let η  be a generic zero of Pi . Then η  annuls every σ-polynomial in P , hence η  is a specialization of η over K. Let ζ and ζ  be subindexings of η and η  , respectively, consisting of coordinates with the same indices. Applying Propositions 4.1.10(ii) and 7.1.1(i), we obtain that σ-trdegL Lη   ≤ σ-trdegK Kη   ≤ σ-trdegK Kη, hence dim Pi ≤ dim P . If the equality holds and ζ  is a σ-transcendence basis of η  over K, then Proposition 7.1.1(ii) implies that Eord Lη  /Lζ  (Pi ) ≤ Eord Kη  /Kζ   ≤ Eord Kη/Kζ.

(7.1.4)

Suppose that the σ-field extensions L/K and Kη/K are incompatible. Then η  is not a generic zero of P and, therefore, not a generic σ-specialization of η over K. Let Σ be a complete set of parameters of Pi . Then ζ  is σ-algebraically independent over L and, hence, over K. We obtain inequalities (7.1.4), the last of which must be strict, as it follows from Proposition 7.1.1(ii). This completes the proof of parts (i) and (ii) of the theorem. Let L/K and Kη/K be compatible. By Corollary 5.1.7, there exists a σ-overfield Lη   of L and a σ-K-isomorphism ρ of Kη into Lη   with the following properties: 1) ρ(η) = η  ; 2) Let ζ be a σ-transcendence basis of η over K. Then ζ  = ρ(ζ) is a σtranscendence basis of η  over L and Eord Lη  /Lζ   = Eord Kη/Kζ. Clearly, ζ  is a solution of some Pi (1 ≤ i ≤ k). Then dim Pi ≥ σtrdegK Kη = dim P . Let Σ be a complete set of parameters of P and let ζ be the corresponding subindexing of η. Since ζ  = ρ(ζ) is σ-algebraically independent over L, Σ is contained in a set of parameters of Pi , hence Σ is a complete set of parameters. It is easy to see that there is a generic zero θ of Pi whose coordinates with the same indices as elements of Σ are the coordinates of ζ  . Then η  is a σspecialization of θ over Lζ  . Applying Proposition 7.1.1 we obtain that Eord (Σ)Pi = Eord Lθ/Lζ   ≥ Eord Lη  /Lζ   = Eord Kη/KζEord (Σ)P, so statement (iii) is proved.

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In order to prove the last statement of the theorem, let Pj be an essential prime divisor of {P }L in L{y} satisfying properties (a) and (b) of statement (iii), and let Q = Pj K{y}. Then Q is a prime σ ∗ -ideal of K{y} containing P and the perfect σ-ideal Q generated by Q in L{y}. It follows that Q contains some essential prime divisor P  of Q . Applying Proposition 7.1.7 and the already proven part of our theorem, we obtain that Eord (Σ)Pj ≤ Eord (Σ)P  ≤ Eord (Σ)Q ≤ Eord (Σ)P . It follows from Proposition 7.1.7(ii) that Q = P . This completes the proof of the theorem.  Exercise 7.1.15 With the notation of the last theorem, give an example of a σ-field extension L/K and proper prime σ ∗ -ideal P of K{y} such that 1 ∈ {P }L . Proposition 7.1.16 Let K be an ordinary difference field with a basic set σ = {α}, let L be a σ-overfield of K, and let K{y} and L{y} be rings of σ-polynomials in σ-indeterminates y1 , . . . , ys over K and L, respectively. Let J be a perfect σ-ideal of the ring K{y} and P1 , . . . , Pk the essential prime divisors of J in K{y}. Then {J}L is the intersection of the essential prime divisors of {Pi }L (1 ≤ i ≤ k). PROOF. Let Qi1 , . . . , Qimi be the esential prime divisors  ofthe perfect σ-ideal {Pi }L in L{y} (1 ≤ i ≤ k). Then {J}L = {P1 · · · Pk }L = mi k # #   {P1 . . . Pk }L = {P1 }L · · · {Pk }L = Qij (see Theorem 2.3.3). Since i=1 j=1



the opposite inclusion is obvious, the proposition is proved.

Theorem 7.1.17 With the notation of Proposition 7.1.16, let P be a prime σ ∗ -ideal of K{y} and let η be a generic zero of P . Furthermore, suppose that L is a σ-overfield of K such that L and Kη are quasi-linearly disjoint over K. (Actually, the last condition refers to the underlying fields of L, Kη and K, respectively, but we have agreed not to use special notation for underlying fields if no confusion can occur.) Then {P }L is a prime σ ∗ -ideal of the same dimension as P . If Σ is a complete set of parameters of P , then Σ is a complete set of parameters of {P }L and Eord (Σ){P }L = Eord (Σ)P . PROOF. For any k ∈ N, let Rk and Sk denote the algebras of polynomials K[{αi yj | 0 ≤ i ≤ k, 1 ≤ j ≤ s}] and L[{αi yj | 0 ≤ i ≤ k, 1 ≤ j ≤ s}], respectively, and let Pk = P Rk . Then Pk is a prime ideal of Rk with generic zero (η, αη, . . . , αk η) = (η1 , . . . , ηs , α(η1 ), . . . , α(ηs ), . . . , αk (ηs )). Furthermore, by Theorem 1.2.42(v), the radical r(Pk Sk ) is a prime ideal of the ring Sk . We denote this radical by Pk . ∞   . It follows that Q = Pk It is easy to see that if A ∈ Pk , then α(A) ∈ Pk+1 k=0

is a prime σ-ideal of L{y}. Let Q be the reflexive closure of Q. Then P ⊆ Q ⊆ {P }L , hence Q = {P }L . Thus, {P }L is a prime σ ∗ -ideal of L{y}.

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401

  By Theorem 1.2.42(iii),  one has Q K{y} = P . Furthermore, Q K{y} is the reflexive closure of Q K{y}. Since the σ-ideal P is reflexive, we obtain  that Q K{y} = P . It follows that if ζ is a generic zero of Q , then there is a σ-K-isomorphism of Kζ onto Kη, hence the σ-field extensions Kη/K and L/K are compatible. The statements about dimension and relative effective orders are direct consequences of Theorem 7.1.14.  Exercise 7.1.18 Use the last theorem to prove the ordinary version of Theorem 5.1.4 without the assumption that the the difference field K is inversive. Corollary 7.1.19 Let K, L, K{y}, and L{y} be as in Proposition 7.1.16 and let the field extension L/K be primary. Furthermore, let P be a prime σ ∗ -ideal of K{y}. Then (i) {P }L is a prime σ ∗ -ideal in L{y} of the same dimension as P . (ii) If Σ is a complete set of  parameters of P , then Σ is a complete set of parameters of {P }L , P = {P }L K{y}, and Eord (Σ){P }L = Eord (Σ)P . PROOF. By Theorem 1.6.40(ii) and Proposition 1.6.49(i), there exists a generic zero η of P such that Kη and L are quasi-linearly disjoint over K. Applying Theorem 7.1.17 we obtain the conclusion of the corollary.  Theorem 7.1.20 Let K be an ordinary difference field with a basic set σ = {α}, L a σ-overfield of K, and K  the algebraic closure of K in L (clearly, K  is a σ-field). Let K{y} and L{y} be the rings of σ-polynomials in σ-indeterminates y1 , . . . , ys over K and L, respectively, and let P be a prime σ ∗ -ideal in K{y}. Furthermore, let P1 , . . . , Pk be essential prime divisors of {P }L in L{y} and let  be essential prime divisors of {P }K
in K  {y}. Then k = m and, P1 , . . . , Pm after a suitable reordering of P1 , . . . , Pk one has (i) dim Pi = dim Pi for i = 1, . . . , k. (ii) Every complete set Σ of parameters of Pi is a complete set Σ of parameters of Pi , and Eord (Σ)Pi = Eord (Σ)Pi (1 ≤ i ≤ k).  (iii) Pi = Pi K  {y} (1 ≤ i ≤ k). PROOF. By Corollary 7.1.19, each {Pi }L is prime σ ∗ -ideal of the ring L{y}. Since {P }L = {{P }K
}L , one can apply Proposition 7.1.16 to obtain k # that {P }L = {Pi }L . i=1   If 1 ≤ i, j ≤ k and i = j, then {Pi }L K  {y} = Pi and {Pj }L K  {y} = Pj , hence {Pi }L  {Pj }L . It follows that {Pi }L , 1 ≤ i ≤ k, are the essential prime divisors of {P }L in L{y} and coincide (up to the order) with P1 , . . . , Pk . The other conclusions of the theorem follow from Corollary 7.1.19.  Theorem 7.1.21 Let K be an ordinary difference field with a basic set σ = {α}, K ∗ the inversive closure of K, K{y} and K ∗ {y} the rings of σ-polynomials in σ-indeterminates y1 , . . . , ys over K and K ∗ , respectively, and P a prime σ ∗ -ideal of K{y}. Then

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(i) {P }K ∗ is a prime σ ∗ -ideal of K ∗ {y}. (ii) P and {P }K ∗ have the same complete sets of parameters and equal effective orders with respect to those sets.  (iii) {P }K ∗ K{y} = P . PROOF. Let A, B ∈ K ∗ {y} and AB ∈ {P }K ∗ . Then there exists m ∈ N such that αm (A), αm (B) ∈ K{y} and the σ-polynomial αm (A)αm (B) is obtained from elements of P with the use of shuffling and taking linear combinations over K (see the description of the construction of a perfect σ-ideal considered after Definition 2.3.1). Since the ideal P is perfect, αm (A)αm (B) ∈ P . Then αm (A) ∈ P or αm (B) ∈ P , hence A ∈ {P }K ∗ or B ∈ {P }K ∗ . It follows that the σ ∗ -ideal {P }K ∗ is prime. Since the compatibility condition required in the assumptions of Theorem 7.1.14(iii) is obviously satisfied, the other statements of our theorem follow from Theorem 7.1.14(iii). 

7.2

Existence Theorem for Ordinary Algebraic Difference Equations

The following fundamental result is an abstract form of existence theorem for ordinary algebraic difference equations. This theorem was first formulated and proved by R. Cohn in [28] where he used the technique of characteristic sets of ordinary difference polynomials. We present here another proof obtained by R. Cohn, the proof he used in his book [41]. It is based on the technique of difference kernels developed in Section 5.2. (Historically, the first attempt to obtain an existence theorem for an algebraic difference equation was made by J. Ritt [162] who considered the case of difference polynomial of first order. Despite the fact that J. Ritt did not obtain a satisfactory general result, his idea of using the technique of characteristic sets developed in [165] (where they are called “basic sets”) appeared to be very fruitful for the study of abstract algebraic difference equations of any order. In particular, it has led to the development of the contemporary technique of characteristic sets of partial difference polynomials, which is an efficient tool in the study of abstract algebraic difference equations.) Theorem 7.2.1 Let K be an ordinary difference field with a basic set σ, K{y1 , . . . , ys } the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and A an irreducible σ-polynomial in K{y1 , . . . , ys }. Then (i) The variety M(A) has principal components. (ii) Suppose that A contains a transform of some yi (1 ≤ i ≤ s). Then (a) If A contains a transform of order 0 of some σ-indeterminate yj and M is a principal component of M(A), then ord (y1 , . . . , yi−1 , yi+1 , . . . , ys )M = ordyi A.

7.2. EXISTENCE THEOREM

403

(b) Let M1 , . . . , Mk be the principal components of M(A) and let d and e denote, respectively, the degree and reduced degree of A with respect to the highest transform of yi which appears in A. Then k 

ld (y1 , . . . , yi−1 , yi+1 , . . . , ys )Mj ≤ d

and

j=1 k 

rld (y1 , . . . , yi−1 , yi+1 , . . . , ys )Mj = e.

j=1

(c) If d1 and e1 denote, respectively, the degree and reduced degree of A with respect to the lowest transform of yi contained in A, then k 

ild (y1 , . . . , yi−1 , yi+1 , . . . , ys )Mj ≤ d1

and

j=1 k 

irld (y1 , . . . , yi−1 , yi+1 , . . . , ys )Mj = e1 .

j=1

(d) Let q = Eordyk A (1 ≤ k ≤ s) and let M be a component of M(A) such that {y1 , . . . , yk−1 , yk+1 , . . . , ys } is a complete set of parameters of M and Eord (y1 , . . . , yk−1 , yk+1 , . . . , ys )M = q. Then M a principal component of M(A). PROOF. We divide the proof in two parts. The first part is devoted to the proof of statement (i) and statements (a) - (c) of (ii) in the case of inversive difference field K. The second part completes the proof of the theorem in the general case. PART I. Suppose, first, that the σ-field K is inversive and for every i = 1, . . . , s, some transform of yi appears in A. Without loss of generality we can assume that A contains transforms of order zero of some σ-indeterminates yj . (Otherwise, A can be replaced by an appropriate σ-polynomial of the form α−k (A) where k is a positive integer.) Thus, without loss of generality, we assume that A is of order zero with respect to y1 , . . . , yp (1 ≤ p ≤ s) and of positive order with respect to every yj with p < j ≤ s. We are going to reduce the problem of solving the equation A = 0 to finding a realization of certain kernel R = (K(a0 , a1 ), τ ) of length one. The following is a description of R. Suppose, first, that p < s, that is, A is not of order zero with respect to every yj (1 ≤ j ≤ s). Let mi = ordyi A for i = p + 1, . . . , s and let (1) (m
) m = p + mp+1 + · · · + ms . We introduce a0 and a1 as m -tuples (a0 , . . . , a0 ) (1) (m
) and (a1 , . . . , a1 ), respectively and say that the first p coordinates of a0 are assigned, respectively, to y1 , . . . , yp , next mp+1 coordinates of a0 are assigned, respectively, to yp+1 , αyp+1 , . . . , αmp+1 −1 yp+1 , . . . , the last ms coordinates of

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a0 are assigned, respectively, to ys , αys , . . . , αms −1 ys . Similarly, we say that the first p coordinates of a1 are assigned, respectively, to αy1 , . . . , αyp , next mp+1 coordinates of a1 are assigned, respectively, to αyp+1 , α2 yp+1 , . . . , αmp+1 yp+1 , . . . , the last ms coordinates of a1 are assigned, respectively, to αys , α2 ys , . . . , αms ys . (i) (i) Now we define the isomorphism τ sending a0 to a1 (1 ≤ i ≤ m ) as follows. (j) First, if a coordinate a1 of a1 is assigned to some αk yi and also some coordinate (j
) (j) (j
) a0 of a0 is assigned to αk yi , then we set a1 = a0 . Let Y = {yi | 1 ≤ i ≤ p} ∪ {αk yj | p + 1 ≤ j ≤ s, 0 ≤ k ≤ mj − 1} and for every αk yj ∈ Y (including the case k = 0 in which we obtain elements in {yi | 1 ≤ i ≤ p}), let bij denote the coordinate of a0 assigned to αj yi , or the coordinate of a1 assigned to αj yi if no coordinate of a0 corresponds to this term. To complete the description of τ (that is, a description of a0 and a1 ) it remains to define bij . Since A is irreducible, (A) is a prime ideal of the ring K[Y ] consisting of multiples of A. Let us define bij as coordinates of an mtuple which is a generic zero of (A). Then A vanishes when every term αk yj in A is replaced by its assigned coordinate of a0 or a1 . Let us show that the coordinates of a0 are algebraically independent over K. Indeed, if it is not so, then the coordinates of a0 annul some polynomial B ∈ K[Y ] which does not contain any of the terms αmi yi , p + 1 ≤ i ≤ s. Then B is annulled by bij whence B is a multiple of A. However, this is impossible, since A contains terms αmi yi . Similarly, using the fact that A contains y1 , . . . , yp , we obtain that the coordinates of a1 are algebraically independent over K. Now we can define τ as an isomorphism of K(a0 ) onto K(a1 ) which maps each component of a0 to the corresponding component of a1 and whose restriction on K coincides with α. The kernel (K(a0 , a1 ), τ ) will be denoted by R. Now, let us consider the case p = s, that is, assume that ordyi A = 0 for (1) (s) i = 1, . . . , s. Let a0 = (a0 , . . . , a0 ) be a generic zero of the ideal (A) in K[Y ] (i) (a coordinate a0 , 1 ≤ i ≤ s, is assigned to yi ). Then we define R to be the kernel of length zero formed by the field K(a). It is easy to see that the solutions of the equation A = 0 are determined by realizations of the kernel R. (If ζ is a realization of R, then a solution of the equation A = 0 can be obtained by equating each yi , 1 ≤ i ≤ s, to the coordinate of a0 assigned to yi ; and all solutions of the equation can be obtained in this way.) Let M1 , . . . , Md be the varieties over K whose generic zeros are the distinct principal realizations of the kernel R we have constructed. Obviously, every Mi , 1 ≤ i ≤ d, is contained in M(P ). We are going to show that M1 , . . . , Md are principal components of M(P ), while all other irreducible components of M(P ) are singular. Suppose that ζ is a principal realization of R which gives a generic zero of a principal component M of M(P ). Then ζ is a regular realization of R. Indeed, if it is not, then M annuls a σ-polynomial B ∈ K{y1 , . . . , ys } such that / (A). Taking i such that yi appears in ordyi B ≤ ordyi A for i = 1, . . . , s and B ∈ A, let us consider the resultant R of A and B with respect to yi (in this case A

7.2. EXISTENCE THEOREM

405

and B are treated as polynomials in several variables of the form αk yj with yi among them). Clearly, R = 0 and Eordyi R < Eordyi A. Applying Proposition 7.1.2 we obtain that either y1 , . . . , yi−1 , yi+1 , . . . , ys is not a set of parameters of M or Eord(y1 , . . . , yi−1 , yi+1 , . . . , ys )M < Eordyi A that contradicts the fact that M is a principal component of M(P ). Since ζ is a regular realization of R, it is a principal realization. (If it is not, then Theorem 5.2.10(v) implies that s − 1 = σ-trdegK Kζ < δR. This is, however, impossible, since a principal realization of R gives a solution of A = 0 and hence, by Proposition 7.1.2, generates a σ-field extension of K of σ-transcendence degree less than s. By Theorem 5.2.10(i), we should have δR < s.) Now we are going to prove that M1 , . . . , Md are principal components of M(P ) that satisfy conditions (a) - (c) in part (ii) of the theorem. Let M denote one of the Mi (1 ≤ i ≤ d), let ζ be the realization of R from which M is obtained, and let η be the generic zero of M consisting of the s coordinates of ζ corresponding to the coordinates of a0 assigned to the yi . Let us construct a special set b0 for R. If p < s, we choose k ∈ N, p + 1 ≤ k ≤ s and define b0 to be a subindexing of a0 consisting of components assigned to y1 , . . . , yp and αmi −1 yi with i = p + 1, . . . , s, i = k. The coordinates of a0 and b1 are the elements bij other than bkmk used in the construction of a0 and a1 at the beginning of the proof. Since αmk yk appears in A, an argument used in the construction of the kernel R shows that these coordinates form an indexing over K whose coordinates are algebraically independent over K. It follows that coordinates of b1 are algebraically independent over K(a0 ). On the other hand, our construction of the elements bij shows that bkmk is algebraically dependent on the set {bij | (i, j) = (k, mk )} over K. Therefore, all bij , and hence all coordinates of a1 , are algebraic over K(a0 , b1 ). Thus, b0 is a special set for R. (If p = s, any s − 1 coordinates of a0 form a special set.) Since our special set has s − 1 coordinates, δR = s − 1, hence dim M = s − 1 (see Theorem 5.2.10(i)). Let us show that any s − 1 σ-indeterminates in the set {y1 , . . . , ys } form a set of parameters of M. (The coordinates of a0 assigned to such a set of parameters may not form a special set, as it happens when 1 ≤ p ≤ s and the omitted coordinate is some yk with k ≤ p.) To prove this, let us take any k ∈ {1, . . . , s} and consider A as a σ-polynomial in one σ-indeterminate yk with coefficients in the σ-ring K{y1 , . . . , yk−1 , yk+1 , . . . , ys }. Since these coefficients are not multiples of A, they do not vanish when all yj are replaced by ηj . It follows that ηk is σ-algebraically independent over Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs , hence the elements η1 , . . . , ηk−1 , ηk+1 , . . . , ηs form a σ-transcendence basis of Kη1 , . . . , ηs  over K. Applying Proposition 7.1.2 we obtain that that ordyk A and Eordyk A are upper bounds for ord(y1 , . . . , yk−1 , yk+1 , . . . , ys )M and Eord(y1 , . . . , yk−1 , yk+1 , . . . , ys )M, respectively. This determines ordyk A and Eordyk A for 1 ≤ k ≤ p. Now suppose that p < s (that is, A is not of order zero in every yi , 1 ≤ i ≤ s) and let k ∈ {p + 1, . . . , s}. Let c0 be a subindexing of a0 consisting of the coordinates assigned to terms αj yi with k ∈ N, 1 ≤ i ≤ s, i = k. Then c0

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

contains a special set. The s − 1 coordinates of c1 assigned to αy1 , . . . , αyp and to αmi yi with p + 1 ≤ i ≤ s, i = k, form an indexing e0 whose coordinates are algebraically independent over K(c0 ) and also over K(a0 ). By the construction, the remaining coordinates of c1 are equal to the corresponding coordinates of c0 , whence trdegK(c0 ) K(c0 , c1 ) = s − 1. The definition of e0 and Proposition 1.6.31(iii) imply that trdegK(c0 ,c1 ) K(a0 , c1 ) = trdegK(c0 ,e0 ) K(a0 , e0 ) = trdegK(c0 ) K(a0 ) . Since coordinates of a0 are algebraically independent over K and mk of these coordinates do not belong to c0 , trdegK(c0 ) K(a0 ) = mk . It follows that ordc0 R is defined and ordc0 R = trdegK(c0 ,c1 ) K(a0 , a1 ) = trdegK(c0 ,c1 ) K(a0 , c1 ) = mk . By Theorem 5.2.10(ii), ord(y1 , . . . , yk−1 , yk+1 , . . . , ys )M = trdegK η1 ,...,ηk−1 ,ηk+1 ,...,ηs  Kη = trdegK c0  Kζ = ordc0 R = mk . Let us show that, without loss of generality, we can assume that Eordyk A = ordyk A. Indeed, let q = Eordyk A and t = mk − q. Replacing each αi yk in A with αi−t yk we obtain a σ-polynomial A such that ordyk A = Eordyk A = q. Now we can construct a kernel R for A as the kernel R was constructed for A. Dropping the coordinates of ζ assigning to yk , αyk , . . . , αt−1 yk one obtains a regular realization ζ  of R . Since σ-trdegK Kζ   = σ-trdegK Kζ = s − 1 and δR = s − 1, ζ  is a principal realization of R (see Theorem 5.2.10(v)). This realization gives a solution η  = (η1 , . . . , ηk−1 , ηk+1 , . . . , ηs ) of A . Furthermore, because of the equality Eord Kη/Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs  = Eord Kη  /Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs , the consideration of the effective order of A can be replaced by the study of the effective order of A . Also, applying Corollaries 4.3.6 and 4.3.8, we obtain that ld(Kη/Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs ) = ld(Kη  /Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs ) and rld(Kη/Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs ) = rld(Kη  /Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs ). Thus, A can be used instead of A in the proof of part (b), so in the following discussion of (b) we assume that Eordyk A = ordyk A, that is, mk = q. Let u0 be the subindexing of a0 consisting of coordinates assigned to y1 , . . . , yk−1 , yk+1 , . . . , ys and let v0 denote the subindexing
of a0 consisting of coordinates assigned to the terms of the set{y1 , . . . , yp } {αmi −1 yi | p + 1 ≤ i ≤ s, i = k}. As before, let c0 be the subindexing of a0 consisting of coordinates assigned to the set of all transforms of yi with i = k. Then u0 ⊆ c0 , v0 ⊆ c0 , coordinates of u0 are algebraically independent over K(a1 ), and v0 is a special set.

7.2. EXISTENCE THEOREM

407

As we have seen, ordc0 R is defined and trdegK(c0 ,c1 ) K(a0 , a1 ) = q. By Theorem 5.2.8(i), for any positive integers r, n and for any s-tuples a0 , . . . , ar+n obtained by successive prolongations of the kernel R, one has trdegK(cr ,...,cr+n ) K(ar , . . . , ar+n ) = q. Repeatedly applying Theorem 5.2.7(ii), we obtain that the set Ur =

r−1 

ui is

i=0

algebraically independent over K(ar , . . . , ar+n ), whence trdegK(cr ,...,cr+n ,Ur ) K(ar , . . . , ar+n , Ur ) = q. By Theorem 5.2.4, the set Vr+n =

∞ 

vi is algebraically independent over

i=r+n+1

K(ar , . . . , ar+n ) and, therefore, over K(ar , . . . , ar+n , Ur ). Since

∞ 

ci coincides

i=0

with the union of Ur , Vr+n , cr , . . . , cr+n , we have q = trdegK(cr ,...,cr+n ,Ur ,Vr+n ) K(ar , . . . , ar+n , Vr+n ) ∞  K(a , . . . , a , ci ). = trdegK(
∞ r r+n i=0 ci ) i=0

The fact that n is any positive integer implies that K( trdegK(K(
∞ i=0 ci )

∞ 

i=0

ci

∞ 

aj ) = q,

j=r

and since r is also an arbitrary positive integer, we obtain that Eord Kη/Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs  = q. By Proposition 7.1.9, η is a generic zero of a principal component of M(A). Thus, the varieties M1 , . . . , Md are principal components of M(A) and satisfy condition (a). Considering the subindexing v0 of a0 , one can easily see that dv0 R and rdv0 R are defined and are the degree and the reduced degree, respectively, of A in αmk yk . Let w0 be the family of coordinates of ζ that correspond to coordinates of v0 . Since ld(Kη/Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs  = ld(Kη/Kw0  and rld(Kη/Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs  = rld(Kη/Kw0  (as it follows from Corollaries 4.3.6 and 4.3.8), the inequality and equality in part (b) follow from Theorem 5.2.10(iii) for yi with p + 1 ≤ i ≤ s. Similarly one can obtain property (b) when ordyi A = 0 for all i = 1, . . . , s. Let m = max{mp+1 , . . . , ms } and let K  denote the inverse difference field of K, that is, the field K treated as a difference field with the basic set σ  = {α−1 }. (Recall that this concept was introduced in Section 4.3 (before Exercises 4.3.11)

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

and used in the definition of inverse limit degree and inverse reduced limit degree of a difference field extension). Let K  {z1 , . . . , zs } be the ring of difference polynomials in difference indeterminates z1 , . . . , zs over K  , and let A be the difference polynomial in this ring obtained from A by the substitution of αm−i zk for the αi yk (1 ≤ k ≤ s, 0 ≤ i ≤ mi for i = p + 1, . . . , s). Let L be a σoverfield of K and let L denote the inverse field of the inversive closure of L. (Clearly, L can be considered as a difference overfield of K  .) If an s-tuple η = (η1 , . . . , ηs ) with coordinates in L is a generic zero of a principal component M of M(A), then αm η = (αm (η1 ), . . . , αm (ηs )) is a generic zero of a principal component M of M(A ). Indeed, it is clear that αm η annuls A , and, whenever λ is a subindexing of the coordinates of η and λ the corresponding subindexing of the coordinates of αm η, we have σ-trdegK Kλ = σ  -trdegK
K  λ  and Eord Kη/Kλ = Eord K  αm η/K  λ . It is easy to see that we have obtained an one-to-one correspondence between principal components of M(A) and principal components of M(A ). Furthermore, this correspondence preserves the relative inverse limit degrees and inverse reduced limit degrees of the corresponding principal components. Applying the part of (b) which has already be proven for A , we obtain (c) except for those yi for which ordyi A > 0 but Eordyi A = 0. In these cases, however, the statements (b) and (c) are equivalent, as it follows from Proposition 4.3.12, and it is easy to check that either (b) or (c) holds in each of such a case. This completes Part I of the proof. Part II. Now we do not assume that the σ-field K is inversive. Part II(a). Let us still suppose that for every i = 1, . . . , s, some transform of yi appears in A. Let K ∗ denote the inversive closure of K. Since the σ-polynomial A is irreducible in the ring K{y1 , . . . , ys }, each irreducible factor of A in K ∗ {y1 , . . . , ys } contains precisely those αj yi which appear in A. Using equality (1.6.2) we obtain that the sum of reduced degrees in any αj yi of the distinct irreducible factors is the reduced degree of A in αj yi ; also the sum of degrees in αj yi of the distinct irreducible factors is less than or equal to degαj yi A. Let η be an s-tuple with coordinates in a σ-overfield of K. Then σtrdegK ∗ K ∗ η = σ-trdegK Kη, and for any subindexing λ of coordinates of η, Eord K ∗ η/K ∗ λ = Eord Kη/Kλ (since the effective orders are defined in terms of the inversive closures and (Kη)∗ = (K ∗ η)∗ , (Kλ)∗ = (K ∗ λ)∗ ). It follows that η is a generic zero of a principal component of the variety M(A) of A over K if and only if η is a generic zero of a principal component of the variety of an irreducible factor of A in K ∗ {y1 , . . . , ys }. Clearly s-tuples η  , η  are generic zeros of distinct varieties over K if and only if they are generic zeros of distinct varieties over K ∗ . (Indeed, if B is a σ-polynomial in K ∗ {y1 , . . . , ys } and just one of η  , η  annuls B, then for sufficiently large r ∈ N, αr (B) lies in K{y1 , . . . , ys } and is annulled by just one of η  , η  .) Thus, the generic zeros of the principal components of the variety of A as a σ-polynomial in K{y1 , . . . , ys } are the same as the generic zeros of the principal components of the irreducible factors of A in K ∗ {y1 , . . . , ys }. Also, no two such factors share a principal component. (Such a component would annul the resultant R of the factors with

7.2. EXISTENCE THEOREM

409

respect to the highest transform of ys that appears in A. Since R = 0 and Eordys R < Eordys A, this is impossible.) Statements (b) and (c) for A now follow from the already proven corresponding statements for the irreducible factors of A in K ∗ {y1 , . . . , ys }. To prove (a) suppose that A contains a transform of order 0 of some yj (1 ≤ j ≤ s). Then, with η and λ as before, we have ord Kη/Kλ ≥ ord K ∗ η/K ∗ λ (this is just the inequality in Proposition 1.6.31(iii)). Therefore, if M is a principal component of M(A) and 1 ≤ i ≤ s, then ord (y1 , . . . , yi−1 , yi+1 , . . . , ys )M ≥ ordyi A. Applying Proposition 7.1.2 we obtain that the last inequality is actually an equality. This completes the proof of (a) and the whole theorem with the restriction stated at the beginning of Part II of the proof. Part II(b). Now we are going to remove the last restriction and assume that there are σ-indeterminates yi such that no transform of yi appears in A. Let yj1 , . . . , yjt be all σ-indeterminates yj such that some transform of yj appears in A and let A˜ denote the σ-polynomial A considered as an element of the ring of σ-polynomials K{yj1 , . . . , yjt }. Furthermore, let ξ = (ξ1 , . . . , ξs−t ) be a ˜ and, let θ be an indexing with generic zero of a principal component of M(A) s − t coordinates which are σ-algebraically independent over Kξ. Then the coordinates of ξ and θ arranged in proper order form an s-tuple η in M(A). Since σ-trdegK Kξ = t − 1, we have σ-trdegK Kη = s − 1. Furthermore, if λ is a subindexing of ξ, then any set of elements of the form αj ξi (1 ≤ i ≤ s − t, j ∈ N) algebraically independent over Kλ is algebraically independent over Kλ, θ. Therefore, for any k ∈ N, any transcendence basis of the set {αj ξi | j ≥ k} over Kλ is also a transcendence basis of this set over Kλ, θ. Since Kη = Kλ, θξ and Kξ = Kλξ, we obtain that ord Kη/Kλ, θ = ord Kξ/Kλ (if one takes k = 0) and Eord Kη/Kλ, θ = Eord Kξ/Kλ (if we take k to be sufficiently large). Furthermore, it follows from Theorem 1.6.28(v) that ld(Kη/Kλ, θ) = ld(Kξ/Kλ) and the corresponding equalities hold for the reduced, inverse, and inverse reduced limit degrees. Therefore, η is a generic zero of a principal component of M(A), and to verify (a), (b), and (c) it is sufficient to show that any generic zero of a principal component of M(A) can be obtained in the same way as we obtained η in the above considerations. In other words, given such a generic zero η, one should show that the subindexing ξ of η corresponding to yj1 , . . . , yjt is ˜ and the subindexing θ consisting a generic zero of a principal component of A, of the remaining components η is σ-algebraically independent over Kξ. Since ξ is a solution of A˜ and θ has s − t coordinates, σ-trdegK Kξ < t and σ-trdegK ξ Kξ, θ ≤ s − t. Also, σ-trdegK ξ Kξ, θ + σ-trdegK Kξ = σtrdegK Kη = s − 1 whence σ-trdegK ξ Kξ, θ = s − t and σ-trdegK Kξ =

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

t − 1. It follows that the coordinates of θ are σ-algebraically independent over Kξ, and ξ satisfies the condition on the σ-transcendence degree for a ˜ stated in Proposition 7.1.9. generic zero of a principal component of M(A) If λ is any subindexing of ξ, then one can proceed as before and obtain that Eord Kη/Kλ, θ = Eord Kξ/Kλ, so that ξ satisfies condition (7.1.3). Applying Proposition 7.1.9 we complete the proof of parts (a) - (c) of our theorem in the general case. To prove part (d) we first notice that the justification presented at the beginning of the proof allows one to assume that K is inversive, Eordyk = ordyk and p < s. (One can easily modify the proof for p = s; we leave this modification to the reader as an exercise.) Let η be a generic zero of M (we use the notation of statement (d) of the theorem). If M is not a principal component of M(A), then η is not a regular realization of the kernel R (see Theorem 5.2.10(v)). In this case η annuls some polynomial B in the set of indeterminates {αj yi |, 1 ≤ i ≤ p, j = 0, 1 or p + 1 ≤ i ≤ s, j = 0, . . . , mi } such that A does not divide B. Let R be the resultant of A and B with respect to αq yk . Then R = 0 and Eordyk R < Eordyk A if q > 0. If q = 0, then the resultant R1 of R and α(A) with respect to αyk is a nonzero polynomial in y1 , . . . , yk−1 , yk+1 , . . . , ys . Since η annuls R and R1 , we arrive at a contradiction with the result of Proposition 7.1.2. This completes the proof of the theorem.  Definition 7.2.2 With the notation of Theorem 7.2.1, let q = Eordyi A and let M be a component of M(A) such that {y1 , . . . , yi−1 , yi+1 , . . . , ys } is a complete set of parameters of M and Eord (y1 , . . . , yi−1 , yi+1 , . . . , ys )M = q. Then M is called a principal component of M(A) with respect to yk . (By Theorem 7.2.1(d), M is a principal component of M(A). ) R. Cohn [28] suggested the following procedure for determining the principal components of M(A) with respect to some yk , 1 ≤ k ≤ s. (We keep the above notation; in particular A is still an irreducible σ-polynomial in K{y1 , . . . , ys }.) First, one should replace each yi in A with an element ηi such that the set {ηi | 1 ≤ i ≤ s, i = k} is σ-algebraically independent over K. Let A denote the resulting σ-polynomial in the σ-polynomial ring Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs {yk } and let L denote the inversive closure of Kη1 , . . . , ηk−1 , ηk+1 , . . . , ηs . Applying the procedure described in Example 5.2.6 we can find principal components of the irreducible factors of A . If ηk is a generic zero of such a principal component, then (η1 , . . . , ηs ) is a generic zero of a principal component of M(A) with respect to some yk . Unfortunately, this procedure does not determine whether the principal components with respect to different yi are identical. The following statement is an application of Theorem 7.2.1 to the description of proper irreducible varieties of maximal dimension. Theorem 7.2.3 Let K be an ordinary difference field with a basic set σ = {α}, K{y1 , . . . , ys } the algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys

7.2. EXISTENCE THEOREM

411

over K, and M an irreducible variety over K{y1 , . . . , ys }. The the following conditions are equivalent. (i) dim M = s − 1. (ii) M is a principal component of the variety of an irreducible σ-polynomial in K{y1 , . . . , ys }. PROOF. Since the implication (ii) ⇒ (i) is obvious, we just need to show that if M is an irreducible variety of dimension s − 1, then there is an irreducible σ-polynomial A ∈ K{y1 , . . . , ys } such that M is a principal component of M(A). Without loss of generality we can assume that {y1 , . . . , ys−1 } is the set of parameters of M. Then every nonzero σ-polynomial in the σ-ideal Φ(M) contains some transform of ys . Let η = (η1 , . . . , ηs ) be a generic zero of M. By Corollary 4.1.20, if q = Eord(y1 , . . . , ys−1 )M, then ηs annuls a nonzero σ-polynomial of effective order q in Kη1 , . . . , ηs−1 {ys } and therefore some σ-polynomial C of effective order q in K{η1 , . . . , ηs−1 }{ys }. Replacing every ηi in C with yi (1 ≤ i ≤ s − 1) we obtain a σ-polynomial B ∈ Φ(M) whose effective order in ys is q. Let A be an irreducible factor of B which lies in Φ(M). Then A contains some transform of ys , Eordys A ≤ q, and M is contained in an irreducible component M of M(A), since a generic zero of M lies in one of such components. By the first part of Proposition 7.1.7, dim M = s − 1, while the second part of the same Proposition leads to the inequality Eord(y1 , . . . , ys−1 )M ≥ q. Now Proposition 7.1.2 implies that the last inequality is actually an equality, so one can apply the last statement of Theorem 7.2.1 and obtain that M is a principal component  of M(A). By Proposition 7.1.7, M = M. Generally speaking, it is not true that limit degrees of the different principal components of the variety of an irreducible difference polynomial are equal (see Exercise 4.3.27). At the same time we have the following statement. Proposition 7.2.4 Let K be an ordinary difference field with a basic set σ = {α}, let K{y} be an algebra of σ-polynomials in one σ-indeterminate y over K, and let A be an irreducible σ-polynomial in K{y} of order 0. Then, if any component of the variety M(A) has limit degree 1, then all components of M(A) are of limit degree 1. PROOF. Let M1 , . . . , Mr be the components of M(A) and let η (1) , . . . , η (r) (i) (i) be their generic zeros, respectively. (η (i) = (η1 , . . . , ηs ) for i = 1, . . . , r; as (i) (i) usual, by a transform αj (η (i) ) we mean the s-tuple (αj (η1 ), . . . , αj (ηs ). ) Since Eord A = 0, all components Mi are principal ones. For every i = 1, . . . , r, let Ni denote the normal closure of the field Ki = K(η (i) , α(η (i) ), α2 (η (i) ), . . . ) over K. As the splitting field over K of the system A, α(A), . . . regarded as polynomials in indetermiantes y, α(y), . . . , respectively (see Theorem 1.6.13(iii) ), the fields Ni are K-isomorphic. By Proposition 4.3.12(ii), ld(Kη (i) /K) = 1 if and only if Ki : K < ∞. It remains to notice that Ki /K is finite if and only if Ni /K is finite (see Theorem 1.6.13(iv)) and  the last condition is satisfied for every Ni or for none.

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Exercise 7.2.5 Show that statement similar to Proposition 7.2.4 hold for the reduced limit degree, inverse limit degree, and inverse reduced limit degree.

7.3

Existence of Solutions of Difference Polynomials in the Case of Two Translations

Unfortunately, the result of Theorem 7.2.1 cannot be extended to the case of partial difference polynomials. The following example is due to I. Bentsen [10]. Example 7.3.1 As in Example 5.1.17, let us consider Q as a difference field with a basic set σ = {α1 , α2 } where α1 and α2 are the identity automorphisms. Let b and i denote the positive square root of 2 and the square root of −1, respectively (we assume b, i ∈ C), and let us treat F = Q(b, i) as a σ-overfield of Q where the extensions of α1 and α2 to automorphisms of F are defined as follows: α1 (b) = −b, α1 (i) = i, α2 (b) = b, α2 (i) = −i. It is easy to show that the σ-polynomial A = y 2 − b in one σ-indeterminate y over F is irreducible in F {y} and A has no solutions. Indeed, if η is a solution of A, then η 2 = b which, as demonstrated in Example 5.1.17, is impossible. The following theorem is a version of the existence theorem for the case of difference polynomials over an inversive difference field with two translations. Theorem 7.3.2 (Bensten). Let K be an inversive difference field with a basic set σ consisting of two translations. Let R = K{y1 , . . . , ys } be the ring of σpolynomials in σ-indeterminates y1 , . . . , ys over K. Furthermore, suppose that there exists a translation α ∈ σ such that if K is treated as an ordinary difference field with the basic set σ  = σ \ {α} (we denote this σ  -field by K  ), then every two σ  -field extensions of K  are compatible. Then every irreducible σ-polynomial A ∈ R\K has a solution ζ = (ζ1 , . . . , ζs ) with the following properties. (i) ζ is not a proper specialization over K of any solution of A. (ii) If A contains a transform of some yk (1 ≤ k ≤ s), then elements ζ1 , . . . , ζk−1 , ζk+1 , . . . , ηs are σ-algebraically independent over K. Furthermore, if a σ-polynomial B ∈ R \K is annulled by η and contains only those transforms of yk which are contained in A, then B is a multiple of A. PROOF. Let α and β denote the translations of K (so that σ = {α, β}). Without loss of generality we can assume that K  in the condition of the theorem is the field K treated as a difference field with the basic set σ  = {α} and the σ-polynomial A has the following property: there exist p, q ∈ {1, . . . , s} such that some transforms of the form αi yp and β j yq (i, j ∈ N) appear in A. (Since K is inversive, one can apply an appropriate transform αd β e (d, e ∈ Z) to A and obtain a σ-polynomial with the described condition.) We say that a difference polynomial with this property is in standard position.

7.3. EXISTENCE THEOREM. THE CASE OF TWO TRANSLATIONS 413 Let us start with the case with the following condition: for every k = 1, . . . , s, some transform of yk appears in A. Let W denote the set of all transforms αi β j yk (i, j ∈ N, 1 ≤ k ≤ s) which appear in A. Furthermore, for every k ∈ {1, . . . , s}, let mk denote the maximum value of j such that αi β j yk ∈ W for some i. Finally, we denote by h an integer between 0 and s such that mk = 0 for k ≤ h and mk > 0 for k > h. Assuming, first, that h < s, consider the set Y = {β j yk | k ≤ h; j = 0, 1} ∪ {β j yk | k > h, 1 ≤ j ≤ q} with t =

s 

(mk + 1) elements. Let us order the elements of Y according

k=2h+1

to the lexicographic order with respect to (k, j) (we denote the elements of Y arranging according to this order by zν , 1 ≤ ν ≤ t): z1 = y1 , z2 = βy1 , z3 = y2 , . . . , z2h = βyh (so that β j yk = z2k−1+j for k = 1, . . . , h; j = 0, 1), z2h+1 = yh+1 , z2h+2 = βyh+1 , . . . , z2h+(mh+1 +1) = β mh+1 yh+1 , . . . , zt = β ms ys (so that k−1  β j yk = zν with ν = k + h + j + mi if k > h, 0 ≤ j ≤ mk ). Denoting i=h+1

the indexing (zν )1≤ν≤t by z we obtain an ordinary difference polynomial ring K  {z} with the basic set σ  = {α} (z1 , . . . , zt are σ  -indeterminates in this ring). Clearly, W ⊆ K  {z} and A can be considered as a σ  -polynomial in K  {z}. Let ξ = (ξ1 , . . . , ξt ) be a generic zero of a principal component of A over K  {z} (the existence of such a generic zero follows from Theorem 7.2.1). Let Z0 be the subindexing of z consisting of all coordinates zν except the coordinates that are equal to βyk with k ≤ h or β mj yj with j > h. Furthermore, let Z1 be the subindexing of z consisting of all coordinates zν except the coordinates that are equal to yk (1 ≤ k ≤ s). Notice that both Z0 and Z1 have l = t − s coordinates and the union of their sets of coordinates is the set of coordinates of z. Furthermore, with respect to the ordering of coordinates of Z0 and Z1 induced by the ordering of coordinates of z, if the νth coordinate of Z0 is β j yk , then νth coordinate of Z1 is β j+1 yk (1 ≤ ν ≤ s). Let ai = (ai1 , . . . , ail ) (i = 0, 1) denote the indexing obtained from z by replacing all coordinates belonging to Zi by the corresponding coordinates of ξ. Then each of the indexings a0 and a1 is σ  -algebraically independent over K  . Indeed, if r ∈ {h + 1, . . . , s}, then there exists p ∈ N (p depends on r) r−1 such that αp β mr yr appears in A. Setting m = r + h + mr + i=h mi , we obtain that the term αp zm appears in A. Therefore, by the definition of ξ, it follows that ξ1 , . . . , ξm−1 , ξm+1 , . . . , ξt are distinct and form a σ  -algebraically independent set over K  . Since m is not in the domain of Z0 , we obtain that a0 is σ  -algebraically independent over K  . Since A is in standard position in the ring K{y1 , . . . , ys }, there exists k  ∈
{1, . . . , s} such that for some p ∈ N (p depends on k  ), the term αp yr
appears
r −1      in A; that is, for m = 2r − 1 if r ≤ h or m = p + h + mi if r > h, i=h

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the elements ξ1 , . . . , ξm
−1 , ξm
+1 , . . . , ξt are distinct and form a σ  -algebraically independent set over K  . By the definition of Z1 , it follows that a1 is also σ  algebraically independent over K  . Since a0 and a1 are σ  -algebraically independent over K  , the automorphism β of K can be extended to an isomorphism τ of K  a0  onto K  a1  such that τ (αi a0ν ) = αi a1ν for i = 0, 1, 2, . . . , 1 ≤ ν ≤ l. (As usual, the extension of the translation α of K  to any σ  -overfield of K  is denoted by the same letter α.) It is easy to see that τ is actually a σ  -isomorphism of K  a0  onto K  a1  which contracts to β on K  . Furthermore, τ (a0ν ) = a1ν , 1 ≤ ν ≤ l. Thus, for the case h < s we have a difference kernel R = (K  a0 , τ ) for A which is similar to the difference kernel for the irreducible ordinary difference polynomial in the proof of Theorem 7.2.1. If h = s, then A can be considered as an ordinary σ  -polynomial in the ring K  {y1 , . . . , ys }. Letting a0 = (a01 , . . . , a0s ) be a generic zero of a principal component of the variety of A over K  {y1 , . . . , ys } and setting τ = β (in this case we view this mapping as a σ  -automorphism of K  ) we again have the desired kernel R = (K  a0 , τ ). The fact that every two σ  -field extensions of K  are compatible implies that the kernel R of length 1 constructed above satisfies the stepwise compatibility condition. Furthermore, the compatibility condition on K  trivially implies the stepwise compatibility condition on the kernel of length 0. Therefore, in either case, Theorems 6.1.12 and 6.2.4 imply that there exists a principal realization η of the kernel R. Since for each k ∈ {1, . . . , s}, there exists a coordinate a0νk of a0 which is assigned to yk , a solution for A over K{y1 , . . . , ys } can be obtained by assigning to yk (1 ≤ k ≤ s) the coordinate of η which corresponds to a0νk . Let us denote this solution by ζ = (ζ1 , . . . , ζs ) (with ζk = ηνk ). Let B be a σ-polynomial in K{y1 , . . . , ys }\K which contains only terms from the set W and B is annulled by η. Since η is a regular realization of R, one can treat B as a σ  -polynomial in K  {z} which is annulled by ξ. Since A is in standard position, this σ-polynomial contains a σ-indeterminate of the form β j yr = zµ . If the resultant R over K of A and B with respect to β j yr is not zero, then R is a nontrivial σ-polynomial containing terms from the set W excluding β j yr , and R is annulled by ξ. Then either ξ1 , . . . , ξµ−1 , ξµ+1 , . . . , ξt are σ  -algebraically dependent over K  or these elements are σ  -algebraically independent over K  (in particular, they are distinct) and Eord K  ξ/K  ξ1 , . . . , ξµ−1 , ξµ+1 , . . . , ξt  < Eordzµ A. In either case we obtain a contradiction with the definition of ξ. It follows that R = 0, and, since A is irreducible, B is a multiple of A. Recall that for every k ∈ {1, . . . , s}, the σ-polynomial A contains some transform of yk and is annulled by ζ. Furthermore, if A is considered as a σ-polynomial in yk with coefficients in K{y1 , . . . , yk−1 , yk+1 , . . . , ys }, then, as we have seen, these coefficients are not annulled by ζ1 , . . . , ζk−1 , ζk+1 , . . . , ζs . It follows that the element ζk is σ-algebraic over Kζ1 , . . . , ζk−1 , ζk+1 , . . . , ζs  for 1 ≤ k ≤ s. Let us show that the elements ζ1 , . . . , ζk−1 , ζk+1 , . . . , ζs are σ-algebraically independent over K. To prove this, we consider the kernel R = (K  a0 , τ ) constructed above, and define a subindexing b0 of a0 as follows. If h < k ≤

7.3. EXISTENCE THEOREM. THE CASE OF TWO TRANSLATIONS 415 s, then b0 = (a01 , . . . , a0h , a0,h+mh+1 , a0,h+mh+1 +mh+2 , . . . , a0,h+mh+1 +···+mk−1 , a0,h+mh+1 +···+mk+1 , . . . , a0,h+mh+1 +···+ms ); if k ≤ h ≤ s, then b0 = (a01 , . . . , a0,k−1 , a0,k+1 , . . . , a0h , a0,h+mh+1 , a0,h+mh+1 +mh+2 , . . . , a0,h+mh+1 +···+ms ). In the first case, τ (b0 ) = b1 is a subindexing of a1 which is σ  -algebraically independent over K  a0  (it follows from the definition of ξ and the fact that A contains αi β mk yk for some i, so one can apply Theorem 6.2.12(i)). In the second case, since some term of the form β i yk appears in A, the coordinates of b0 are σ  -algebraically independent over K  a1  if h < s, and if h = s, the coordinates of b0 are σ  -algebraically independent over K  . Applying Theorem 6.2.12(ii) we obtain that the subindexing θ of η corresponding to b0 is σ-algebraically independent over K. Now let us show that ζ is not a proper specialization over K of any solution of A. Suppose that λ = (λ1 , . . . , λs ) is a solution of A such that λ (σ-) specializes over K to the s-tuple ζ. Furthermore, suppose that h < s and consider the l-tuple ω = (ω1 , . . . , ωl ) of those transforms of coordinates of λ which are substituted into the coordinates of Z0 with ων substituted into the νth coordinate of Z0 (1 ≤ ν ≤ l). Let ω1 be defined in the same way as ω, with the Z0 replaced by Z1 . We construct a kernel R by contracting the translation β of Kλ to a σ  -isomorphism of K  ω onto K  ω1 . With the obvious correspondence of coordinates, η is a specialization of ω over K. Therefore, with the appropriate identification of coordinates indicated in the definitions of z, Z0 and Z1 , (η, η1 ) is a specialization of (ω, ω1 ) over K  . The last specialization is generic, since (η, η1 ) is equivalent to a generic zero of a principal component of the variety of A over K  . It follows that ω is a regular realization of R in Kλ. Applying Theorem 6.2.9 we obtain that η is a generic specialization of ω over K, hence ζ is a generic specialization of λ over K. The case h = s can be treated in the same way except for the notation changes. Now we can complete the proof by removing the restriction that each yk (1 ≤ k ≤ s) has a transform which appears in A. In this case one can proceed as in Part II(b) of the proof of Theorem 7.2.1, with λ interpreted as a solution of the kind whose existence has been established above for the restricted case. Using Lemma 6.2.7 one easily obtains that ζ is not a proper specialization over K of any other solution of A. Then the verifications of properties (i) and (ii) of the theorem are straightforward (we leave the details to the reader as an exercise).  Corollary 7.3.3 Let a difference field K with two translations and an irreducible difference polynomial A in K{y1 , . . . , ys } be as in Theorem 7.3.2. Then A has at most finitely many nonequivalent solutions of the type described in the theorem. PROOF. By Theorems 2.5.11 and 2.6.3, K{y1 , . . . , ys } is a Ritt difference ring and the variety M({A}) of the perfect σ-ideal of K{y1 , . . . , ys } generated by A has a unique irredundant representation as the union of finitely many irreducible varieties. Any solution of A which is not a proper specialization of any other solution of A is a generic zero of one of these irreducible components

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of M({A}). It remains to notice that all generic zeros for each such component of M({A}) are equivalent.  Corollary 7.3.4 Let K be an inversive difference field with two translations and let K be algebraically closed or separably algebraic closed. Then every irreducible difference polynomial A of positive degree over K has a solution that has properties (i) and (ii) of Theorem 7.3.2. PROOF. Applying Theorem 5.1.6 we obtain that the field K satisfies the hypothesis of Theorem 7.3.2.  A weakening of the hypothesis of Theorem 7.3.2 leads to the following result. Theorem 7.3.5 Let K be a difference field whose basic set σ consists of two translations α and β, and let K{y1 , . . . , ys } be the ring of σ-polynomials in σindeterminates y1 , . . . , ys over K. Then the following conditions are equivalent. (i) K has an algebraic closure which is a σ-field extension of K. (ii) Every algebraically irreducible σ-polynomial A ∈ K{y1 , . . . , ys } \ K has a solution η with the properties stated in Theorem 7.3.2. (iii) If a is any element separably algebraic and normal over K, then K may be extended to a σ-field Ka. PROOF. (i)⇒ (ii). Let L be a σ-overfield of K which is also an algebraic closure of K. By Proposition 2.1.9(i), the inversive closure L∗ of L is algebraically closed. Let A ∈ K{y1 , . . . , ys } \ K be an irreducible σ-polynomial and let B be a nontrivial irreducible factor of A in L∗ {y1 , . . . , ys }. Furthermore, let λ = (λ1 , . . . , λs ) be a solution of B of the type whose existence is established in Corollary 7.3.4. Then λ is a solution of A. Let M denote an irreducible component of the variety M(A) of the σpolynomial A over K{y1 , . . . , ys } which contains λ, and let η = (η1 , . . . , ηs ) be a generic zero of M. Then η is a solution of A which specializes to λ over K, and η is not a proper specialization over K of any other solution of A. It is easy to see that if a term αi β j yk appears in A, it appears in B as well. Therefore, for every k ∈ {1, . . . , s}, such that some transform of yk appears in A, the elements λ1 , . . . , λk−1 , λk+1 , . . . , λs are σ-algebraically independent over L∗ (see Corollary 7.3.4) an hence over K. It follows that the elements η1 , . . . , ηk−1 , ηk+1 , . . . , ηs are also σ-algebraically independent over K. Let W be the set of all terms αi β j yk that appear in the σ-polynomial A and let C ∈ K{y1 , . . . , ys } \ K be a σ-polynomial annulled by η and containing only terms from W . Let us show that the resultant of A and C over K with respect to any term αp β q yr ∈ W is identical zero. Indeed, let R be such a resultant. Then R is annulled by η and hence by λ. Applying Corollary 7.3.4 we obtain that R, considering as a nontrivial σ-polynomial over L∗ , is a multiple of B. This is, however, impossible, since B contains αp β q yr and R does not. Thus, R = 0. It follows that A and C have a common divisor in K{y1 , . . . , ys } \ K. Since A is irreducible in K{y1 , . . . , ys }, C is a multiple of A.

7.3. EXISTENCE THEOREM. THE CASE OF TWO TRANSLATIONS 417 (ii) ⇒ (iii) is obvious. (iii) ⇒ (i). Since every finitely generated separably algebraic field extension is simple (see Theorem 1.6.17), condition (iii) implies the following fact: if a1 , . . . , am are any elements of an overfield of K such that each ai , 1 ≤ i ≤ m, is separably algebraic and normal over K, then one can extend K to a σ-field Ka1 , . . . , am . Let us show that condition (iii) implies that K can be extended to a σ-field defined on the separable closure of K. (If the field K is perfect (in particular, if Char K = 0), this will complete the proof.) Let S be the separable part of an algebraic closure K of K, and let N denote the family of finite normal field extensions of K in S. Let us choose one primitive (over K) element for each extension in N, and let Z be this collection of elements of S. Let X be a set of elements σ-algebraically independent over K such that Card X = Card Z, and let the one-to-one correspondence zi  xi between the elements of Z and X be fixed. Furthermore, for every xi ∈ X, let Ai (xi ) denote a zero order σpolynomial in K{X} (that is, Ai do not contain any nontrivial transforms of elements of X) such that Ai , when regarded as an algebraic polynomial over K, is a minimal polynomial of zi over K. The system of all such σ-polynomials Ai (xi ) ∈ K{X} will be denoted by F. Let us show that if the system F has a solution over K, then there is a structure of a σ-overfield of K on the field S. To prove this, suppose F has a solution ξ = {ξi }. Then the field Kξ is separably algebraic over K. Indeed, if η ∈ Kξ, then there exists a field K  between K and Kξ generated over K by finitely many transforms of elements of ξ and such that η ∈ K  . Since every ξi is separably algebraic over K, any transform αp β q ξi (p, q ∈ N) is separably algebraic over αp β q (K) and hence also over K. It follows that K  /K is a separably algebraic field extension, hence η is separably algebraic over K. Thus we may without loss of generality assume that K ⊆ Kξ ⊆ S. In order to complete the proof of our statement about the difference structure on S, we are going to show that S = Kξ. Indeed, if ζ ∈ S, then K(ζ)/K has a finite separable normal closure N/K in S/K. Then there exists an element zi ∈ Z which generates N over K, that is, N is a splitting field over K for Ai (xi ). By the uniqueness of a splitting field (see Theorem 1.6.5), we have K(ζ) ⊆ K(ξi ) ⊆ Kξ. Thus, S is contained in Kξ whence S = Kξ. In order to complete the proof of the implication (iii)⇒(i) (and hence the proof of the theorem) we need the following generalization of Theorem 2.6.5 to the case of difference polynomial rings with arbitrary (not necessarily finite) set of difference indeterminates. Lemma 7.3.6 Let K be a difference field with a basic set σ and let U be a family of elements in some σ-overfield of K such that U is σ-algebraically independent over K. Let Φ be any system of σ-polynomials in the σ-polynomial ring K{U }. Then the following statements are equivalent. (a) Φ has a solution in some σ-overfield of K. (b) 1 ∈ / {Φ}. (As usual, {Φ} denotes the perfect ideal of K{U } generated by Φ.)

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

PROOF. Since the implication (a) ⇒ (b) is obvious, it is sufficient to prove that (a) implies (b). Suppose that 1 ∈ / {Φ}. By Zorn’s lemma, there exists a maximal proper perfect ideal P of K{U } containing {Φ}. It is easy to see that P is a prime reflexive difference ideal. (If a, b ∈ P and a ∈ / P, b ∈ / P , then we can apply Theorem 2.3.3 and arrive at a contradiction: 1 ∈ {P, a}, 1 ∈ {P, b}, but 1∈ / {P, a}{P, b}, since {P, a}{P, b} ⊆ {P, ab} ⊆ P .) Then K can be considered as a σ-subfield of the quotient σ-field L of K{U }/P , and the set U of residue classes of elements of U is a generic zero of Φ in L (and hence a solution of Φ).  COMPLETION OF THE PROOF OF THE THEOREM Let us show that condition (iii) of the theorem implies that the system F has a solution. Indeed, if it is not so, then the perfect ideal {F} generated by F coincides with the whole ring K{X}. Therefore, there exists a finite subset X  ⊆ X such that the subset Φ of Φ corresponding to X  generates the whole ring K{X  }. By the above lemma, we obtain that Φ is a finite set of irreducible, normal, separable difference polynomials over K which has no solutions in any σ-overfield of K. This is, however, impossible, as it is explained in the first paragraph of the proof (i) ⇒ (ii). Thus, condition (iii) implies that Φ has a solution over K and there is a structure of a σ-overfield of K on S. It remains to prove that if Char K = p > 0, then one can extend S to a σ-overfield of K which is an algebraic closure of K. This can be done by introducing a translation τ : a → ap on S. Clearly, τ commutes with α and β on S, and τ maps K into K. Thus, one can consider K and S as difference fields with the basic set σ1 = {α, β, τ } such that S is a σ1 -overfield of K. It is easy to see that the σ1 -inversive closure S ∗ of S is perfect and hence contains a perfect closure S1 of S. Clearly, S1 is an algebraic closure of K and all three translations of S ∗ map S1 into S1 ; in particular S1 can be considered as a σ-overfield of K. This completes the proof of the theorem.  Exercise 7.3.7 Prove that the equivalence of conditions (i) and (iii) of the last theorem holds for difference fields with any (finite) basic sets of translation. The following example shows that there exists a difference polynomial A over an inversive partial difference field K such that the order of A with respect to every translation is positive and A has a solution of the type described in Theorem 7.3.2, though the the field K does not satisfy condition (i) of Theorem 7.3.5 (see Example 5.1.17). Example 7.3.8 (Bentsen). Let K be an inversive difference field with a basic set σ = {α, β} and let A = αy + βy + αβ 2 y be a difference polynomial in the ring of σ-polynomials in one σ-indeterminate y over K. Let K  denote the field K treated as an ordinary difference field with the basic set σ  = {α}, and let a0 = (a00 , a01 , a02 ) be a generic zero of a principal component of the variety of A considered as an ordinary σ  -polynomial in K  {αy, βy, αβ 2 y}. Then there exists a σ  -isomorphism τ of K  a00 , a01  onto K  a00 , a02  such that a00 → a01 and a01 → a02 and whose contraction to K is β. Then R = (K  a00 , a01 , a02 , τ ) is a difference (σ  -) kernel.

7.3. EXISTENCE THEOREM. THE CASE OF TWO TRANSLATIONS 419 By Theorem 7.2.1, ord K  a00 , a01 , a02 /K  a00 , a01  = 1, so that a02 is transcendental over K  a00 , a01 . It follows that the field extension K  a00 , a01 , a02 / K  a00 , a01  is primary, hence the kernel R satisfies property L∗ (see Proposition 6.1.7). By Corollary 6.2.4, R has a principal realization and all principal realizations are equivalent of this kernel are equivalent. If η is a principal realization of R, then η is a solution of A satisfying the properties stated in Theorem 7.3.2. We complete this section with a theorem on the number of generators of a perfect ideal in a ring of difference polynomials over completely aperiodic ordinary difference field. Theorem 7.3.9 (R. Cohn). Let K be a completely aperiodic ordinary difference field with a basic set σ = {α} and let K{y1 , . . . , yn } be the ring of σpolynomials in σ-indeterminates y1 , . . . , yn over K. Then every perfect σ-ideal of K{y1 , . . . , yn } has a basis consisting of n + 1 elements. PROOF. Let Q be a nonzero perfect ideal of K{y1 , . . . , yn } and let A1 , . . . , Ar be nonzero σ-polynomials such that Q = {A1 , . . . , Ar }. (We use the notation of Section 2.3, so for any Φ ⊆ K{y1 , . . . , yn }, {Φ} denotes the perfect ideal generated by the set Φ.) Let us consider (n + 1)r σ-indetermiantes zij (1 ≤ i ≤ n + 1, 1 ≤ j ≤ r) over K{y1 , . . . , yn } and let K{z} denote the corresponding algebra of σ-polynomials over K. Furthermore, let K{y, z} denote the algebra of σ-polynomials in n + nr + r σ-indeterminates zij , yk (1 ≤ k ≤ n) over K. r  zij Aj and show that there is For every i = 1, . . . , n + 1, let us set Bi = i=1

a nonzero σ-polynomial C ∈ K{z} such that every (n + nr + r)-tuple η, which annuls every Bi (1 ≤ i ≤ n + 1), is either a solution of C or a solution of every Aj , 1 ≤ j ≤ r. Let us adjoin to K elements aij , 1 ≤ i ≤ n + 1, 2 ≤ j ≤ r and bi , 1 ≤ i ≤ n, that form a σ-algebraically independent set over K. Then one can find an (n+1)tuple (a11 , . . . , an+1,1 ) as a solution of the σ-polynomial obtained from Bi by setting yj = bj (1 ≤ j ≤ n) and zij = aij (1 ≤ j ≤ r). (The fact that such an (n + 1)-tuple (a11 , . . . , an+1,1 ) exists is the consequence of Theorem 7.2.1.) It is easy to see that the elements a11 , . . . , an+1,1 belong to
the σ-field obtained by adjoining the set {aij | 1 ≤ i ≤ n + 1, 2 ≤ j ≤ r} {b1 , . . . , bn } to K. In what follows we write a, b, y, and z for the indexings (aij )1≤i≤n+1, 1≤j≤r , (b1 , . . . , bn ), (y1 , . . . , yn ), and (zij )1≤i≤n+1, 1≤j≤r , respectively. Let J be the reflexive prime σ-ideal of K{y, z} with generic zero y = b, z = a. Then  Bi ∈ J for i = 1, . . . , n + 1. Since dim J = n + (n + 1)(r − 1) < (n + 1)r, J K{z} contains a nonzero difference polynomial C1 . Successively applying the process of reduction, described in the proof of Theorem 2.4.1, we obtain that there exist a linear combination B of B1 , . . . , Bn+1 with coefficients in K{y, z}, a difference polynomial D, which does not contain transforms of z11 , . . . , zr1 , , and some product I of transforms of A1 such that IC1 − B = D. Then D ∈ J and, moreover, D = 0, since otherwise D could not be annulled by the generic

420

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

zero (y = b, z = a) of the ideal J. Thus, every common solution of B1 , . . . , Bn+1 annuls either C1 or A1 . Similar arguments show the existence of nonzero difference polynomials C2 , . . . , Cr ∈ K{z} such that every common solution of B1 , . . . , Bn+1 annuls either Cj or Aj (2 ≤ j ≤ r). Then C = C1 . . . Cr has the desired property: every common solution of B1 , . . . , Bn+1 annuls either C or every Aj , 1 ≤ j ≤ r. Since the difference field K is completely aperiodic, there exist elements ζij ∈ K (1 ≤ i ≤ n = 1, 1 ≤ j ≤ r) such that B does not become zero if one replaces each zij by ζij . Let Bi become Bζi after this replacement. Then Bζi ∈ Q (1 ≤ i ≤ n + 1). Furthermore, if (y1 , . . . , yn ) = (η1 , . . . , ηn ) is a solution of every Bζi , then yi = ηi , zij = ζij (1 ≤ i ≤ n + 1, 1 ≤ j ≤ r) is a common solution of B1 , . . . , Bn+1 not annulling C. Therefore, (η1 , . . . , ηn ) annuls every Aj , 1 ≤ j ≤ r. The last observation shows that the difference polynomials Bζi ∈ Q (1 ≤  i ≤ n + 1) generate Q as a perfect difference ideal of K{y1 , . . . , yn }.

7.4

Singular and Multiple Realizations

In what follows we apply the results of Section 7.2 to the study of non-principal realizations of difference kernels over ordinary difference fields. Definition 7.4.1 A realization η of a difference kernel R over an ordinary difference field K is called singular if η is not a specialization (over K) of a principal realization of R. A realization of R is called multiple if it is a specialization of two non-isomorphic principal realizations of R. The following two examples are due to R. Cohn [41, Chapter 6, Section 21]. Example 7.4.2 Let K be an ordinary difference field with a basic set σ = {α}. Let us consider an algebraically irreducible σ-polynomial A = (α2 y)y + αy in the ring of σ-polynomials K{y} in one difference indeterminate y over K. Then yα(A) − A = αy( (α3 y)y − 1). If η = 0 is a generic zero of an irreducible component M of M(A), then η must annul (α3 y)y − 1, whence (α3 y)y − 1 ∈ Φ(M), 0∈ / M. Thus, the solution 0 of A itself constitutes an irreducible component of M(A). Clearly, it is a singular component and furnishes a singular realization of the kernel produced for A by the procedure described in Example 5.2.6. Example 7.4.3 As in the preceding example, let K{y} be the ring of difference polynomials in one difference indeterminate y over an ordinary difference field K with a basic set σ = {α}. Let A = (αy)2 + y 2 ∈ {y}. Since α(A) − A = (α2 y − y)(α2 y + y), the variety M(A) has two principal components M1 and M2 which annul α2 y − y and α2 y + y, respectively. Let η be a generic zero of M1 and let x be an element in some overfield of Kη which is transcendental over Kη. Then the field L = Kη(x) can be treated as a σ-overfield of Kη if we set α(x) = x. Clearly, the element xη ∈ L is a solution of A which is not algebraic over K. Therefore, xη is contained

7.4. SINGULAR AND MULTIPLE REALIZATIONS

421

in a principal component of M(A). Since xη is not a solution of α2 y + y, this element belong to M1 . Furthermore, since 0 is a solution of any σ-polynomial with solution xη, 0 ∈ M1 . Similar arguments show that 0 ∈ M2 . We obtain that 0 is a multiple realization of the kernel R formed for A as in the proof of Theorem 7.2.1. The following theorem of R. Cohn is a fundamental result on singular and multiple realizations. Theorem 7.4.4 Let R = (K(a0 , . . . , ar ), τ ) be a difference kernel over an ordinary inversive difference field K of zero characteristic with a basic set σ = {α}. Let S denote the polynomial ring in s(r + 1) indeterminates K[{αj yi |1 ≤ i ≤ s, 0 ≤ j ≤ r}] ⊆ K{y1 , . . . , ys }, and let P be the prime ideal of S with the general zero (a0 , . . . , ar ). Then there exists a σ-polynomial A ∈ S \ P such that (a) Every singular realization of R annuls A. (b) If r = 0, then R has no singular realization (even if Char K = 0). (c) Every multiple realization of R annuls A. (d) Every regular realization of R is a specialization of one and only one principal realization. PROOF. Using Remark 5.2.12 we can replace R by a kernel of length 0 or 1. We start with the case of a kernel R of length 1: R = (K(a0 , a1 ), τ ). Let b0 be a special set for R (see the corresponding definition before Theorem 5.2.8) and let c0 be a maximal subindexing of the coordinates of a0 = (1) (s) (a0 , . . . , a0 ) algebraically independent over K(b0 ). Then W = b0 ∪ b1 ∪ c0 is (j) algebraically independent over K, and every coordinate ai (i = 0, 1; 1 ≤ j ≤ s) is algebraic over K(W ). (i ) (i ) In what follows, for any subindexing dk = (ak 1 , . . . , ak p ) of the indexing (1) (s) ak = (ak , . . . , ak ) (k = 0, 1) and for any j ∈ N such that k ≤ 1 − j, dkj (ip ) (i1 ) will denote the subindexing (ak+j , . . . , ak+j ) of ak+j . Furthermore, d∗k will denote the subindexing (αk yi1 , . . . , αk yip ) of the indexing (αk y1 , . . . , αk ys ). If Σ ⊆ K{y1 , . . . , ys }, then Σj (j ∈ N) will denote the set {αj (f ) | f ∈ Σ}. Finally, we set S = K[y1 , . . . , ys , αy1 , . . . , αys ] (this agrees with the notation in the condition of the theorem) and Sk = K[{αj yi | 1 ≤ i ≤ s, 0 ≤ j ≤ k + 1}] for every k ∈ N (S0 = S). (i) / b0 ∪ c0 for some i ∈ {1, . . . , s}, then the ideal P contains an irreIf a0 ∈ (i) ducible polynomial Bi = 0 which lies in K[b∗0 , c∗0 , yi ]. Also, if a0 ∈ c0 \ b0 , then P contains an irreducible polynomial Ci = 0 which lies in K[b∗0 , b∗1 , c∗0 , αyi ]. By Theorem 1.2.44, there is a polynomial D ∈ S \ P such that if a pair of s-tuples η0 , η1 is a solution of P not annulling D, then P has solution of the form (i)

(i)

αk yi = fk + ηk (i)

(k = 0, 1; 1 ≤ i ≤ s) (i)

(7.4.1)

where the elements fk which correspond to ak ∈ W form an algebraically independent set B over the field K  = K(η0 , η1 ), while the remaining elements (i) (ν) fk are series in positive integral powers of the fµ ∈ B with coefficients in K  .

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

422

Let A be the product of D and the formal partial derivatives ∂Bi /∂yi and ∂Cj /∂(αyj ) for all i, j for which Bi and Cj are defined. These partial derivatives are not in P , since the resultant of Bi and ∂Bi /∂yi with respect to yi and the resultant of Cj and ∂Cj /∂(αyj ) with respect to αyj are nonzero polynomials in / P . We are going to show that A satisfies condition K[b∗0 , b∗1 , c∗0 ]. Therefore, A ∈ (a) of the theorem. Let η be a realization of the kernel R not annulling A. Then there is a (i) / B, then its series expansion solution of P of the form (7.4.1). Indeed, if f0 ∈ (j) does not contain elements f1 which lie in B. Applying Theorem 1.2.44 one (i) (i) (i) obtains a solution of Bi of the form yi = f¯0 + η0 where f¯0 is a series in (j) (i) (i) powers of some f0 . Since the solution is unique by Theorem 1.2.44, f¯0 = f0 . (i) (i) In what follows, we define a transform α(f0 ) (written also as αf0 ) as (j) a formal power series obtained by replacing each f0 occurring in the power (i) (j) (i) series for f0 by a symbol g1 and each coefficient c in the series for f0 by (i) (i) (i) (i) α(c). (If f0 ∈ B, then α(f0 ) = g1 .) Furthermore, let (f0 ) denote the series (j) (i) (i) (i) obtained by replacing each g1 in α(f0 ) by the series for f1 . (If f1 ∈ B, this (i) replacement is just f1 itself.) (i) (i) (i) Let us show that (f0 ) = f1 . Indeed, if f0 ∈ B, the equality is obvious. (i) Let f0 ∈ / B. Then Bi is defined and α(Bi ) ∈ P ∩ K[b∗1 , c∗1 , αyi ]. If we replace (j) (j) each αyj , j = i, with the series for f1 + η1 , we shall obtain a polynomial B i in αyi with power series coefficients. Since ∂(αBi )/∂αyi is not annulled by (j) the η1 , it follows from Theorem 1.2.45 that B i has at most one solution for (i) αyi as a sum of η1 and a series in positive integral powers of some elements (i) of B. Since α(Bi ) is annulled by (7.4.1), this solution is f1 . Furthermore, by (i) Theorem 1.2.44, α(Bi ) has a unique formal solution for αyi as a sum of η1 (j) and a series in positive integral powers of the remaining αyj − η1 occurring in (i) (j) (j) α(Bi ). This series is α(f0 ), except that αyj − η1 is written for g1 for each (j) (j) j. Replacing every αyj − η1 in this series by the series for f1 one obtains (i) the unique series solution f1 of B i . On the other hand, the result of such a (i)  (i) (i) replacement is (f0 ) whence (f0 ) = f1 . (i) (i) (i) For every i such that a0 ∈ b0 we define fk (k = 2, 3, . . . ) so that these fk (1) (s) (1) (s) form an algebraically independent set L over K  (f0 , . . . , f0 , f1 , . . . , f1 ). (i) (i) Furthermore, for every i such that a0 ∈ / b0 we define inductively the set fj (j = 2, 3, . . . ) as series in powers of elements of finite subsets of B∪L considered (i) as parameters. Suppose that for some integer k ≥ 2, the fj have been defined (i)

(µ)

for all j < k. As before, we define α(fj ) by replacing formally each fν (i)

series for fj

(µ)

in the

(i)

with gν+1 , and each coefficient c with α(c). Then (fj ) is formed (µ)

(µ)

(µ)

by replacing every gν+1 with the series for fν+1 . In any case either fν+1 ∈ B ∪ L (i) (i) or ν = 0, so that these series have already been defined. We set fj+1 = (fj ) . (i)

(i)

(Thus, we obtain that fj+1 = (fj ) for j = 0, 1, . . . .)

7.4. SINGULAR AND MULTIPLE REALIZATIONS

423 (i)

(i)

Let E be a σ-polynomial in K{y1 , . . . , ys } which is annulled by the fj +ηj , (i)

(i)

then α(E) is annulled by the substitution of the α(fj )+ηj+1 for the αj+1 yi , and (i)

(i)

therefore by the substitution of the (fj ) + ηj+1 for the αj+1 yi . It follows that (i)

(i)

the substitution of the fj + ηj annuls α(E), so that the set of σ-polynomials (i)

(i)

of K{y1 , . . . , ys } annulled by the substitution of the fj + ηj is a prime σ-ideal Q containing P .  Let us show that Q K[c∗i ; b∗i , b∗i+1 , . . . ] = (0) for i = 0, 1, . . . . We proceed (µ) by induction on i. If i = 0, the result follows from the construction of fν . Suppose that the statement is true for all i < j where j > 0. Clearly, the  intersection Q(j) = Q K[αj−1 y1 , . . . , αj−1 ys , αj y1 , . . . , αj ys ] is a prime ideal αj−1 ys , αj y1 , . . . , αj ys ] containing αj−1 (P ). By the of the ring K[αj−1 y1 , . . . ,  (j) K[c∗j−1 , b∗j−1 , b∗j , . . . ] = (0). induction hypothesis, Q

Since the cardinality of the set c∗j−1 b∗j−1 b∗j is the dimension of αj−1 (P ), whence Q(j) = αj−1 (P ). Furwe have the inequality Q(j) ≥ dim αj−1 (P ),   dim ∗ ∗ thermore, since P K[c0 , b0 ] = (0), onehas P K[c∗1 , b∗1 ] = (0). Therefore, αj−1 (P ) K[c∗j , b∗j ] = (0) and, hence, Q K[c∗j , b∗j ] = (0). It follows from the  (i) construction of the fk (k > j) that Q K[c∗i ; b∗j , b∗j+1 , . . . ] = (0). Let us prove that the difference ideal Q is reflexive. Let a finite sequence of s-tuples a0 , . . . , ak+1 be a generic zero of the ideal Qk = Q ∩ Sk in the ring Sk . In accordance with the previous conventions, for every subindexing λ of a0 and for every i = 0, 1, . . . , λi will denote the corresponding subindexing of ai . Letting e0 = c0 ∪ b0 ∪ · · · ∪ bk+1 and using the result of the previous paragraph we obtain that the set e0 is algebraically independent over K. Since a0 , a1 is a generic zero of P , every coordinate of a0 , a1 is algebraic over K(c0 , b0 , b1 ). Since a1 , a2 is a generic zero of P1 = α(P ) considered as an ideal in K[αy1 , . . . , αys , α2 y1 , . . . , α2 ys ], every coordinate of a1 , a2 is algebraic over K(c1 , b1 , b2 ) and also over K(c0 , b0 , b1 , b2 ) (since c1 is a subset of the coordinates of a1 ). Continuing in this way we obtain that e0 is a transcendence basis of the coordinates of a0 , . . . , ak+1 over K. Now, in order to show that the σ-ideal Q is reflexive, we assume that α(F ) ∈ Q for some σ-polynomial F ∈ K{y1 , . . . , ys }\Q and obtain a contradiction. Let k / Qk , every essential prime divibe a positive integer such that F ∈ Sk . Since F ∈ sor of the ideal (F, Qk ) in Sk contains a nonzero polynomial of K[c∗0 ; b∗0 , . . . , b∗k+1 ] (it follows N ∈  from Remark 1.2.39). Therefore, there is a nonzero  polynomial ∗ ∗ ∗ ), Q ) K[c ; b , . . . , b (F, Qk ) K[c∗0 ; b∗0 , . . . , b∗k+1 ]. Then α(N ) ∈ (α(F k+1 1 1 k+2 ].  On the other hand, we have shown that Q K[c∗1 ; b∗1 , . . . , b∗k+2 ] = (0). This contradictionshows that the σ-ideal Q is reflexive. Since Q K[b∗0 , b∗1 , . . . ] = (0), we have dim Q > δR. Furthermore, since Q ∩ K[c∗0 , b∗0 , b∗1 ] = (0), a generic zero of Q is a regular realization of the kernel R. By Theorem 5.2.10(v), a generic zero of Q is a principal realization of R. If (i) (i) F ∈ Q, then F is annulled by the substitution of the fj + ηj for the αj yi .

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

Applying Theorem 1.2.43 we obtain that F is annulled by the substitution of (i) the ηj for the αj yi . Thus, η is a solution of Q which is not a singular realization of R. This completes the proof of statement (a). (It has been proved for kernels of length 1, and the consideration of kernels of length 0 is a trivial particular case of statement (b) proved below.) PROOF OF STATEMENT (b). With the above notation, let a kernel R be given by K(a0 ) and let η be a realization of R. Replacing K by its σ-overfield, if necessary, we assume that η ∈ K and the field K is algebraically closed. Successive generic prolongations of R lead to kernels Rr = (K(a0 , . . . , ar ), τr ), r = 0, 1, . . . , and it is sufficient to prove that one can specialize a0 , a1 , . . . over K to η0 = η, η1 = α(η), . . . . We know that a0 specializes to η0 . Suppose that it has been shown that there is a specialization φ of a0 , . . . , ar to η0 , . . . , ηr = αr (η) (r ≥ 1). Clearly, there is also a specialization ψ of ar+1 to ηr+1 = αr+1 (η). Considering the dimensions one obtains that K(a0 , . . . , ar+1 ) is the free join over K of K(a0 , . . . , ar ) and K(ar+1 ). It follows (see Proposition 1.6.49) that the field extensions K(a0 , . . . , ar )/K and K(ar+1 )/K are quasi-linearly disjoint. Applying Proposition 1.6.65 we obtain that φ and ψ yield a specialization of a0 , . . . , ar+1 to η0 , . . . , ηr+1 , so one has a specialization over K of a0 , a1 , . . . to η0 , η1 = α(η), . . . . PROOF OF STATEMENT (c). As before, we may assume that the kernel R is of length 0 or 1. We shall give the proof for length 1. (The case of length 0 can be considered in a similar way; we leave the corresponding trivial modifications to the reader as an exercise.) Let R = (K(a0 , a1 ), τ ) and let b0 , c0 , and a σ-polynomial A ∈ K{y1 , . . . , ys } be defined as in the proof of part (a). Let η be a realization of R which do not (i) (i) annul A. As before, there is a series fj + ηj (i = 1, . . . , s; j = 0, 1, . . . ) and a reflexive σ-ideal Q of K{y1 , . . . , ys } annulled by the series whose generic zero is a principal realization of R. Let Q be a reflexive prime σ-ideal of K{y1 , . . . , ys } annulled by η whose generic zero is a principal realization of R. We should prove that Q = Q . Suppose that Q = Q . By Theorem 5.2.10, Q and Q have a common complete set of parameters and the same relative order with respect to this set. Furthermore, it follows from Proposition 7.1.7(ii) thatQ  Q , so that there exists k ∈ N and a σ-polynomial H such that H ∈ (Q Sk ) \ Q . By Theorem  (i) (i) 1.2.43(ii), Q Sk has a solution not annulling H of the form αj yi = gj + ηj (i)

(1 ≤ i ≤ s, 0 ≤ j ≤ k + 1) where gj are series in positive integral powers of a parameter t which is transcendental over the field formed by adjoining the  series solution of Q to Kη. Using this solution of Q Sk , one can obtain a  (i) (i) solution αj yi = hj + ηj (1 ≤ i ≤ s, 0 ≤ j ≤ k + 1) of Q Sk as follows. If





(i) (i) αj yi ∈ c∗0 b∗0 · · · b∗k+1 , we set hj = gj ; if αj yi ∈ / c∗0 b∗0 · · · b∗k+1 , then (i)

hj

(i) hj

(µ)

is defined by replacing each fν

(i)

in the series for fj

are series in positive integral powers of t.)

(µ)

with gν . (Clearly,

7.5. REVIEW OF FURTHER RESULTS (i)

(i)

425 (i)

Let us show that gj = hj for all i, j for which hj has been defined. Of


course, it is sufficient to consider i, j such that αj yi ∈ / c∗0 b∗0 · · · b∗k+1 (in the case of inclusion our statement is trivial). Proceeding by induction on k we
observe, first, that if yl ∈ c∗0 b∗0 , then one can define an irreducible polynomial (i) ∗ ∗ the g0 for the yi of Bl which Bl = 0 which
∗ lies in K[c0 , b0 , yl ]. After substituting ∗  are in c0 b0 we obtain a polynomial Bl in yl with power series coefficients. Since ∂Bl /∂yl is not annulled by the η i , Bl has at most one solution for yl as the sum of η l and a series in positive integral powers of t (see Theorem 1.2.45). Since  (l) (l) (l) (l) Bl ∈ Q Q , these series is g0 and also h0 , so that g0 = h0 . Suppose that it has been proved that for some integer m, 0 ≤ m ≤ k + 1, (i) (i) (i) (i) gj = hj (i = 1, . . . , s; j = 0, . . . , m − 1). We are going to show that gm = hm . ∗ ∗ It is clear if yi ∈ b0 . Suppose that yi ∈ c0 . Then one can define the polynomial Ci considered in the first part of the proof, and this polynomial has the property that αm−1 (Ci ) ∈ K[c∗m−1 , b∗m−1 , b∗m , αm yi ]. Replacing the elements of

(µ) (µ) c∗m−1 b∗m−1 b∗m in αm−1 (Ci ) with the corresponding gν , which are also hν by the induction hypothesis and the previous case, we obtain a polynomial Ci in (i) αm yi . It has at most one solution for αm yi as a sum of ηm and a series in positive   (i) m−1 (Ci ) ∈ Q Q , this solution must be gm and also integral powers of t. Since α
(i) (i) hm . Finally, if a0 ∈ b0 c0 , then Bi is defined and αm Bi ∈ K[c∗m , b∗m , αm yi ].
(µ) Replacing the elements of c∗m b∗m in αm Bi with the corresponding gν , which (µ) are also hν by the previous case, we obtain a polynomial Bi in αm yi . Applying (i) (i) (i) (i) the uniqueness argument we obtain that gm = hm . Thus, gj = hj for all i, j (i)

(i)

for whichhj has been defined. We have arrived at a contradiction, since gj annul Q Sk , but these elements do not annul H. This completes the proof of statement (c). The last statement of the theorem follows from the fact that no regular realization of the kernel R can annul the σ-polynomial A.  Remark 7.4.5 The proof of part (b) of the last theorem shows that if R is a kernel of length 0 and η and ζ are two realizations of R generating two compatible σ-field extensions of K, then η and ζ are specializations of the same principal realization. In particular, if K is algebraically closed, then all principal realizations of R are equivalent, so there can be no multiple realization. If K is not algebraically closed, then distinct principal realizations of R generate incompatible field extensions of K. For example, 0 is a multiple realization of the kernel corresponding to the polynomial y 2 + z 2 over Q.

7.5

Review of Further Results on Varieties of Ordinary Difference Polynomials

In this section we review some results on varieties of ordinary difference polynomials. Practically all of these results were obtained in R. Cohn’s works [28], [29], [31], [32], [35], [40], [41, Chapters 8 and 10], [42], [43], and [46], where one can find the detailed proofs.

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In [41, Chapter 8] R. Cohn showed that the solutions of any irreducible variety M over an ordinary difference field K can be obtained by rational operations, transforming, and the inverse of transforming from the solutions of a principal component N of the variety of an algebraically irreducible difference polynomial. (N is a variety over K, but not over the same ring of difference polynomials, as M.) More precisely, R. Cohn proved the following statement. Theorem 7.5.1 Let K be an ordinary difference field with a basic set σ = {α}, K{y1 , . . . , ys } the ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and M an irreducible variety over K{y1 , . . . , ys }. Suppose that K is completely aperiodic or that dim M > 0, and also that M possesses a complete set of parameters {y1 , . . . , yk } such that rld (y1 , . . . , yk )M = ld (y1 , . . . , yk )M (We assume that the σ-indeterminates are so numbered that the first k of them constitute the complete set of parameters. Note also that the last equality always holds if Char K = 0.) Then there exist: (a) an irreducible variety N over the ring of σ-polynomials K{y1 , . . . , yk ; w} (w is the (k + 1)-th σ-indetreminate of this ring); (b) σ-polynomials Ak+1 , . . . , As ∈ K{y1 , . . . , yk }, σ-polynomials Bk+1 , . . . , Bs , / N , and an integer t ≥ 0 such that C ∈ F {y1 , . . . , yk ; w}, C ∈ (i) N is a principal component of the variety of an algebraically irreducible σ-polynomial of K{y1 , . . . , yk ; w}, y1 , . . . , yk constitute a complete set of parameters of N , and Eord (y1 , . . . , yk )N = Eord (y1 , . . . , yk )M. (ii) If (η1 , . . . , ηs ) is any solution in M, there is a solution in N with yi = ηi s  for i = 1, . . . , k, and w given by the result of substituting ηj for yj in Ai yi . i=k+1

(iii) If yi = ζi , i = 1, . . . , k, w = θ is a solution in N which does not annul C, then there is a solution in M with yi = ζi , i = 1, . . . , k and yj , k + 1 ≤ j ≤ s, given by applying α−t to the result of substituting ζk+1 , . . . , ζs , θ for yk+1 , . . . , ys , w, respectively, in Bj /C. (iv) If D ∈ K{y1 , . . . , ys } \ Φ(M), then there exists a σ-polynomial E ∈ F {y1 , . . . , yk ; w} \ Φ(N ) such that any solution in N not annulling E gives rise by a procedure described in (iii) to a solution in M not annulling D. (v) The procedures of (ii) and (iii) carry generic zeros of M or N into generic zeros of N or M. Whenever (iii) is defined, these procedures, applied to elements of M or N , are inverses of each other.  With the notation of the last theorem, W =

s  i=k+1

Ai yi is called a resolvent

for M or for Φ(M), and Φ(N ) is called a resolvent ideal for M or for Φ(M). One can say that M is obtained from the solutions of its resolvent ideal by the relations αt yi = Bi /C (k + 1 ≤ i ≤ s) and the solutions of the resolvent ideal are obtained from those of M by the relation w = W . Let K{y1 , . . . , ys } be a ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over an ordinary difference field K with a basic set σ = {α}. Let A be a σpolynomial in K{y1 , . . . , ys } which contains at least one transform of some yi ,

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and let αm yi and αl yi be the transforms of yi of the highest and lowest orders, respectively, appearing in A. Then the separants of A with respect to yi are defined to be the formal partial derivatives ∂A/∂(αm yi ) and ∂A/∂(αl yi ) (computed with the assumption that A is a polynomial over K in the set of indeterminates αi yj which appear in A). If A is written as a polynomial in αm yi , then the coefficient of the highest power of αm yi in A is said to be the initial of A with respect to yi . The following two theorems, proved in [41, Chapters 6 and 10], summarize basic properties of a variety of one ordinary difference polynomial. Theorem 7.5.2 Let K be an ordinary difference (σ-) field of zero characteristic, K{y1 , . . . , ys } a ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and A an algebraically irreducible σ-polynomial in K{y1 , . . . , ys }. Then (i) Every singular component of the variety M(A) and every solution common to two principal components of M(A) annuls the separants of A. (ii) It is possible to adjoin to K an arbitrarily large number of generic zeros of a principal component M of M(A), except in the case that s = 1 and Eord A = 0. (iii) Let M(A) have only one principal component, let Φ ⊆ K{y1 , . . . , ys } be the prime difference ideal of this component, and let I be an initial of A with respect to some yk . Then a σ-polynomial B belongs to Φ if and only if there is an integer m > 0 and a product J of transforms of I such that (JB)m ∈ [A]. (Therefore, every singular component of M(A) annuls the initials of A.)  Theorem 7.5.3 Let K be an ordinary difference (σ-) field, K{y1 , . . . , ys } a ring of σ-polynomials over K, and A ∈ K{y1 , . . . , ys } \ K an algebraically irreducible σ-polynomial. Then (i) The variety M(A) consists of one or more principal components and (possibly) of singular components. Each component of M(A) has dimension s − 1. (ii) The relative effective orders and (with certain limitations) the relative orders of principal components of M(A) are determined by the effective orders and orders of A. Furthermore, it is sufficient for a component to have dimension s − 1 and one of these effective orders for it to be a principal component. (iii) Except in the case that s = 1 and A is of effective order 0, every principal component of M(A) contains infinitely many generic zeros. (iv) The relative effective orders of the singular components of M(A) are less by at least 2 than those of the principal components. More precisely, if Eordyi A = ri (1 ≤ i ≤ s) and M is a principal component of M(A), then either y1 , . . . , yi−1 , yi+1 , . . . , ys do not constitute a set of parameters of M or they do constitute such a set and Eord (y1 , . . . , yi−1 , yi+1 , . . . , ys )M ≤ ri − 2. (v) Any singular component of M(A) is itself a principal component of M(B) for some algebraically irreducible σ-polynomial B ∈ K{y1 , . . . , ys }. (As it follows from (iv), for each i, 1 ≤ i ≤ s, either B contains no transforms of yi or Eordyi B ≤ ri − 2.)

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(vi) If s = 1 and A is a first order σ-polynomial in R, then M(A) has no singular components.  Theorem 7.5.4 With the assumptions of Theorem 7.5.3, let B and C be σpolynomials in K{y1 , . . . , ys } whose orders with respect to any σ-indeterminate yi (1 ≤ i ≤ s) do not exceed 1. If the variety M({A, B}) is not empty, it has a component of dimension not less than s − 2.  COMPLETE SYSTEMS OF DIFFERENCE OVERFIELDS Let K be a difference field with a basic set σ. A family E of σ-overfields of K is said to be a complete system of difference (or σ-) overfields of K if distinct perfect σ-ideals of any ring of σ-polynomials over K have distinct E-varieties. As we have seen in Section 2.6, the universal system U(K) (see Definition 2.6.1) is complete. R. Cohn ([41, Chapter 8]) constructed a complete system U  (K) of difference overfields of an ordinary difference field K whose members are transformally algebraic extensions of K. Furthermore, if the field K is algebraically closed, then U  (K) may be chosen to consist of a single difference field. Another important result on complete systems of difference overfields is the following criterion for complete systems (see [41, Chapter 8, Section 5]). Theorem 7.5.5 Let K be an aperiodic ordinary difference field of zero characteristic with a basic set σ. Let K{y} be the algebra of σ-polynomials in one σ-indeterminate y over K and let E be a family of σ-overfields of K. Then E is a complete system of σ-overfields of K if and only if given any reflexive prime σ-ideal P of K{y} and any σ-polynomial f ∈ K{y} \ P , the E-variety ME (P ) contains a solution not annulling f .  In what follows we consider an important for analytical applications example of an ordinary difference field of complex-valued functions of real argument. Such a function f (x) is said to be a permitted function if it is defined for x ≥ 0 except at a set S(f ) which has no limit points, is analytic in each of the intervals into which the non-negative real axis is divided by omission of the points of S(f ), and is either identically 0, or is 0 at only finitely many points in any finite interval. A permitted difference ring is an ordinary difference ring R whose elements are permitted functions and whose basic set consists of the isomorphism f (x) → f (x + 1), f (x) ∈ R. (More precisely, elements of R are equivalent classes of permitted functions, with f (x) equivalent to g(x) if and only if f (x) = g(x) for any x ∈ / S(f ) ∪ S(g).) If a permitted difference ring is a field, it is called a permitted difference field. Let K0 be the difference field of rational functions with complex coefficients whose domain is the set of non-negative real numbers and translation α is defined by α(f (x)) = f (x + 1), f (x) ∈ K0 . Then K0 is a permitted difference field with basic set σ = {α}. Let F be the set of all permitted σ-overfields of K0 and let F  = {K ∈ F | there exists an infinite set of functions analytic on [0, 1] which is algebraically independent over the field K[0,1] obtained by restricting the domains of functions of K to [0, 1]}. Considering functions with distinct isolated

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essential singularities outside of [0, 1] one can easily obtain a non-enumerable set of functions analytic on [0, 1] which is algebraically independent over K0 (more precisely, over (K0 )[0,1] ). Therefore, every member of F which is at most countably generated over K0 is a member of F  . Theorem 7.5.6 With the above notation, if K is a σ-field in F  , then F is a complete system of σ-overfields of K.  Theorems 7.5.3 and 7.5.4 imply the following result. Theorem 7.5.7 Let K ∈ F  , let K{y} be the ring of σ-polynomials in one σindeterminate y over K, and let P be a proper reflexive prime σ-ideal of K{y}. Then P has a generic zero in one of the members of F.  Let a complex-valued function f (x) of real variable x be defined on the interval [0, ∞) except at a set S(f ) which has no limit points. Then f (x) is said to be essentially continuous if either it is identically 0 or it is continuous at every 1 = 0 for every s ∈ S(f ). The following result point of [0, ∞) \ S(f ) and lim x→s f (x) describes a case when a prime difference ideal of K0 {y} has a generic zero whose coordinates are essentially continuous functions. Theorem 7.5.8 With the notation introduced before Theorem 7.5.6, let K = K0 and let a reflexive prime σ-ideal P of K0 {y} have order 0. Then P has a generic zero in some difference field F ∈ F such that every function f (x) ∈ F is continuous for all sufficiently large x and essentially continuous.  We conclude this brief review of properties of permitted functions with the following “approximation theorem” obtained in [42]. Let U be a set of permitted functions and let g(x) be a permitted function. Then g(x) is said to adhere to U at a point a ∈ [0, ∞) if the function g(x) is defined at the points a + i (i = 0, 1, 2, . . . ) and for every  > 0 and positive integer t, there exists a function h(x) ∈ U such that h(x) is defined at a, a+1, . . . , a+t and |g(a+i)−h(a+i)| <  for i = 0, 1, . . . , t. Obviously, if a is a point of adherence, so is a + m for every m ∈ N. Theorem 7.5.9 With the above notation, let K ∈ F and let K{y} be the algebra of difference (σ-) polynomials in one σ-indeterminate y over K. (i) Let J be a σ-ideal of K{y} and let U be a set of solutions of J in σoverfields of K belonging to F. Furthermore, let g(x) be a permitted function such that the set of points of adherence of g(x) to U is dense in [0, ∞). Then g(x) is a solution of J. (ii) Suppose that K ∈ F  , and let P be a prime reflexive σ-ideal of K{y}. Furthermore, let g(x) be a solution of P in a member of F containing K and let V be the set of generic zeros of P in members of F containing Kg(x). Then g(x) adheres to V at a set of points dense in [0, ∞).

430

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS WEIGHT CONDITIONS

The following results were obtained by R. Cohn in [32] and [41, Chapter 10]. Let K be an ordinary difference field with a basic set σ and K{z} the ring of σ-polynomials in one σ-indeterminate z over K. By a weight function on K{z} we mean a function u → fu from the set of all terms u = cz i0 (αz)i1 . . . (αr z)ir of K{z} (r, i0 , . . . , ir ∈ N, 0 = c ∈ K) to the ring of polynomials in one real variable t with coefficients in N such that fu (t) = ir tr + · · · + i0 . t is called the weight parameter. More generally, if K{y1 , . . . , ys } is the algebra of σpolynomials in σ-indeterminates y1 , . . . , ys over K, then every term u of this ring (that is, a power product of the σ-indeterminates and their transforms multiplied by a nonzero coefficient from K) can be written as a product of terms u1 , . . . , us where ui is a term in K{yi } (1 ≤ i ≤ s). Then the weight function on K{y1 , . . . , ys } assigns to u a function fu in s + 1 real variables k1 , . . . , ks , t such s  ki fui (t). In this case the variables k1 , . . . , ks , t are that fu (k1 , . . . , ks , t) = i=1

called weight parameters. If a1 , . . . , as , b are real numbers, then fu (a1 , . . . , as , b) is said to be the weight of the tern u for these values of the weight parameters. Clearly, fu (1, . . . , 1) is the total degree of the term u (treated as a monomial in variables αj yi , j ∈ N, 1 ≤ i ≤ s). Also, it is easy to see that if we consider a formal symbol λ and define its transform α(λ) as λt , then the substitutions yi = λki (1 ≤ i ≤ s) convert u to a power of λ whose exponent is fu (k1 , . . . , ks , t). In what follows, for any σ-polynomial A ∈ K{y1 , . . . , ys } and any positive real numbers a1 , . . . , as , b as values of the weight parameters, l(A; a1 , . . . , as , b) and h(A; a1 , . . . , as , b) will denote the σ-polynomials consisting of the terms of least and highest weights of A, respectively. The proof of the following result can be found in [41, Chapter 10]. Theorem 7.5.10 With the above notation, let A and B be two σ-polynomials in K{y1 , . . . , ys }. With positive real numbers a1 , . . . , as , b as values of the weight parameters, if M(A) ⊆ M(B), then M(l(A; a1 , . . . , as , b)) ⊆ M(l(B; a1 , . . . ,  as , b)) and M(h(A; a1 , . . . , as , b)) ⊆ M(h(B; a1 , . . . , as , b)). Let A ∈ K{y1 , . . . , ys } and let M be an irreducible variety over K{y1 , . . . , ys } such that M ⊆ M(A) and dim M = s − 1. Let η = (η1 , . . . , ηs ) be a generic zero of M and let B be an irreducible σ-polynomial such that M is a principal component of M(B). Without loss of generality we can assume that y1 , . . . , ys−1 form a complete set of parameters of M. Let Kη{z} be the ring of σ-polynomials in one σ-indeterminate z over the σ-field Kη and let A¯ and ¯ be the σ-polynomials of this ring obtained from A and B, respectively, by B ¯ are not 0 and substitutions yi = ηi (1 ≤ i ≤ s − 1), ys = ηs + z. Then A¯ and B have common solution z = 0. Definition 7.5.11 With the above notation, we say that the σ-polynomial A satisfies the low weight condition with respect to M for the parametric set y1 , . . . , ys−1 if

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(i) A¯ contains a term whose weight is lower than that of any other term of A¯ for all values of the weight parameter t in the interval (0, 1). (ii) A¯ contains a term whose weight is lower than that of any other term of ¯ A for all values of the weight parameter t in the interval (1, ∞). ¯ ∗ ) where A¯∗ and B ¯ ∗ denote the σ-polynomials consisting (iii) M(A¯∗ ) ⊆ M(B ¯ respectively. of the terms of least degree in A¯ and B, It is easy to see that the fact that A satisfies the low weight condition does not depend on the choice of B. (Of course, B is not uniquely determined: if one replaces B by a σ-polynomial B1 which is a transform of B with a nonzero coefficient from K, then M will still be a principal component of M(B1 ) = M(B).) At the same time, it is not known whether the low weight condition depends on the choice of a set of parameters of M. Theorem 7.5.12 Let K be an ordinary difference field of zero characteristic with a basic set σ and let K{y1 , . . . , ys } be the algebra of σ-polynomials in σindeterminates y1 , . . . , ys over K. Let A ∈ K{y1 , . . . , ys } and let M be an irreducible variety over K{y1 , . . . , ys } such that M ⊆ M(A). Then a necessary condition for M to be an irreducible component of M(A) is that A should satisfy the low weight condition with respect to M for every complete set of parameters of M.  In the case of σ-polynomials of one σ-indeterminate, the last theorem leads to the following statement proved in [35] (see also [41, Chapter 10]). Theorem 7.5.13 Let K{y} be the algebra of difference polynomials in one difference indeterminate y over an ordinary difference field K of zero characteristic. Let A be an irreducible difference polynomial in K{y} of effective order at least 1. Then (i) A necessary condition for {0} to be a singular component of M(A) is that A should contain a term which is of lower weight than any other term of A for every value of the weight parameter in the interval (0, ∞). If the σ-polynomial A consists of two terms, this condition is also sufficient. (ii) Suppose that A admits the solution 0 and contains a term of first degree. If j is the order of the transform of y in this term, then a necessary and sufficient condition for {0} to be an irreducible component of M(A) is that every term of A contain a transform of y of order not exceeding j and a transform of y of order not less than j.  Another application of Theorem 7.5.12 is the following interesting result on singular components of a variety of an irreducible difference polynomial. Theorem 7.5.14 (R. Cohn, [32]) Let K be an ordinary difference field of zero characteristic with a basic set σ = {α}, K{y1 , . . . , ys } the algebra of σpolynomials in σ-indeterminates y1 , . . . , ys over K, and A an irreducible σpolynomial in this algebra. Let αi ys and αj ys be, respectively, the lowest and the highest transforms of ys which appear in A. Then

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(i) Every singular component of the variety M(A) annuls the formal partial derivatives ∂A/∂(αi ys )k and ∂A/∂(αj ys )k , k = 1, 2, . . . . (ii) Let A be written as A = A0 + A1 αi ys + . . . Ap (αi ys )p and A = B0 + B1 αj ys + . . . Bq (αj ys )q where Aν do not contain αi ys , and Bµ do not contain αj ys (1 ≤ ν ≤ p, 1 ≤ µ ≤ q). Then every singular component of M(A) annuls  each Aν and Bµ . Let K be an ordinary difference field with a basic set σ, K{y1 , . . . , ys } the algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and A an irreducible σ-polynomial in this algebra. Let M be an irreducible variety over K{y1 , . . . , ys } contained in M(A) and let η = (η1 , . . . , ηs ) be a generic zero of M. Furthermore, let ri = Eordyi A (1 ≤ i ≤ s) and let A¯ be the σ-polynomial over Kη obtained from A by replacing every yi by yi + ηi . We say that the σ-polynomial A satisfies the high order condition with respect to M if there exist positive values a1 , . . . , as ; b of the weight parameters and an integer i, 1 ≤ i ≤ s such that the polynomial A¯∗ consisting of the terms of A¯ of least weight (for these values of the weight parameters) has an irreducible factor B in Kη{y1 , . . . , ys } such that Eordyi B > ri − 2. (If ri < 2, this condition means ¯ that some transform of yi appears in A.) Theorem 7.5.15 With the above notation, suppose that Char K = 0 and the irreducible σ-polynomial A satisfies the high order condition with respect to M. Then M is contained in a principal component of M(A).  The low weight condition is not a sufficient condition for an irreducible variety to be an irreducible component of the variety of a difference polynomial. R. Cohn [35] presented the following example which justifies this assertion. Let Q be the field of rational numbers treated as an ordinary difference field whose basis set σ consists of the identity automorphism α. Let K = Qa where the element a is a σ-transcendental over Q (a lies in some σ-overfield of Q), and let K{y} be the algebra of σ-polynomials in one σ-indeterminate y over K. R. Cohn proved that if one considers σ-polynomials A = y(α2 y)2 + (αy)2 α3 y − a(αy)(α2 y) and B = y(α2 y)2 + (αy)2 α3 y − (αy)(α2 y) in K{y}, then 0 is an irreducible component of M(A) but not of M(B). Since A and B comprise the same power products, the example shows that the low weight condition is not a sufficient one. Moreover, this example shows that no criterion dependent only on what power products appear in a difference polynomial is a necessary and sufficient condition for 0 to be an irreducible component of the variety of the polynomial. The following theorem, together with Theorem 7.5.12, can be considered as a summary of most important known results on the low weight condition.

7.6. RITT’S NUMBER. GREENSPAN’S AND JACOBI’S BOUNDS

433

Theorem 7.5.16 (R. Cohn, [40]) Let K be an ordinary difference field of zero characteristic with a basic set σ, K{y1 , . . . , ys } the algebra of σ-polynomials in σ-indeterminates y1 , . . . , ys over K, and A an irreducible σ-polynomial in this algebra. Let M be an irreducible variety with parameters y1 , . . . , ys−1 over K{y1 , . . . , ys } such that M ⊆ M(A) and let r = Eord (y1 , . . . , ys−1 )M, k = Eordys A − r. Then (i) If k = 1, A does not satisfy the low weight condition with respect to M for the parametric set y1 , . . . , ys−1 . (ii) If k = 2, the low weight condition for the parametric set y1 , . . . , ys−1 is a necessary and sufficient condition for M to be an irreducible component of M(A). (iii) If k > 2, the low weight condition for the parametric set y1 , . . . , ys−1 is a necessary but not a sufficient condition for M to be an irreducible component of M(A). 

7.6

Ritt’s Number. Greenspan’s and Jacobi’s Bounds

Let K be an ordinary difference field with a basic set σ = {α} and let K{y1 , . . . , ys } be the ring of σ-polynomials in a set of σ-indeterminates Y = {y1 , . . . , ys } over K. Let Φ ⊆ K{y1 , . . . , ys }, Y  ⊆ Y , and let the orders of σ-polynomials of Φ in each yi ∈ Y \ Y  be bounded (in particular, Φ may be a finite family). For every yi ∈ Y \ Y  , let ri denote the maximum  of  the orders of the σ-polynomials of Φ in yi . Then the number R(Y )Φ = ri i

(the summation extends over all values of the index i such that yi ∈ Y \ Y  ) is called the Ritt number of the system Φ associated with the set Y  ⊆ Y . If s  ri and also write RΦ for R(Y  )Φ. Y  = ∅, we set R(Y  )Φ = i=1

Let 0 = A ∈ Φ and let h(A) be the greatest nonnegative integer such that for each i with yi ∈ Y \ Y  , αh(A) (A) is of order at most ri in yi . Let h = max{h(A)|0 = A ∈ Φ}, that is, h is the greatest integer such that some polynomial of Φ may be replaced by its hth transform without altering the Ritt number of Φ. The number G(Y  )Φ = R(Y  )Φ−h is called the Greenspan number of the system Φ associated with the set Y  ⊆ Y . If Y  = ∅, we write GΦ for G(Y  )Φ. Now, let the system of σ-polynomials Φ be finite, Φ = {A1 , . . . , Am }, and let rij = ordyj Ai , 1 ≤ i ≤ m, 1 ≤ j ≤ s (rij is taken to be 0 if no transforms of yj of order ≥ 1 appear in Ai ). If m = s, then the number J (Φ) = max

 s  i=1

 riji |(j1 , . . . , js ) is a permutation of 1, . . . , s

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is called the Jacobi number of the system Φ. If m = s (it can be also applied to the case m = s), the Jacobi number of the system Φ is defined as the Jacobi number of the corresponding m × s-matrix (rij ) (see the definition before Theorem 1.2.47); this matrix will be denoted by AΦ . The following two theorems (where we use the above notation) gives some bounds on the effective orders with respect to Y  that involve the Ritt, Greenspan and Jacobi numbers. The proof of the statements of the first theorem is due to R. Cohn ([41, Chapter 8, Theorems IX and X]); the result on the Jacobi bound (Theorem 7.6.7) was obtained by B. Lando in [116]. Theorem 7.6.1 (i) With the above notation, suppose that the Ritt number R(Y  )Φ for a system of σ-polynomials Φ (and a set Y  ⊆ Y ) is defined. If M is a component of M(Φ) for which Y  contains a complete set of parameters, then Eord (Y  )M ≤ R(Y  )Φ. (ii) If GΦ is defined, M is a component of M(Φ) and Y  contains a complete set of parameters of every component of M(Φ), then Eord (Y  )M ≤ G(Y  )Φ. PROOF. (i) Let us show first that one can assume that Y  = ∅ in the first statement of the theorem. Indeed, suppose that Y  is nonempty and let η be a generic zero of M. Let η  denote the subindexing of η whose coordinates correspond to elements of Y  , and let η ∗ and Y ∗ be the complimentary subindexings to η  in η and Y  in Y , respectively. Let Φ∗ be the set of σ-polynomials in Kη  {Y ∗ } obtained from the σ-polynomials in Φ by replacing each coordinate of Y  by the corresponding coordinate of η  . It is easy to see that η ∗ is a solution of Φ∗ and a specialization of a generic zero η ∗∗ of some component of M(Φ∗ ). (As in Section 5.3, if no confusion can result, we use term “specialization” for a σ-specialization over K.) It follows that η  , η ∗ is a specialization of η  , η ∗∗ over K. Since η  , η ∗∗ is a solution of Φ, and η  , η ∗ is a generic zero of a component of M(Φ), this specialization is generic. Then there is a σ-Kη  -isomorphism of Kη  , η ∗  onto Kη  , η ∗∗ , hence η ∗∗ is a generic zero of some component M∗ of M(Φ∗ ). Clearly, the empty set is a complete set of parameters of M∗ , Eord M∗ = Eord(Y  )M, and RΦ∗ = R(Y  )Φ. Thus, it is sufficient to prove that Eord M∗ ≤ RΦ∗ so from the very beginning we can assume that Y  = ∅. Now we are going to show that one can assume that ri ≤ 1 for i = 1, . . . , s (we use the notation introduced at the beginning of this section). Indeed, suppose that ri > 1 for some i, say r1 > 1. Let K{Y, z} be the ring of σ-polynomials in one σ-indeterminate z over K{Y } = K{y1 , . . . , ys }. Let Φ be a family of σpolynomials in K{Y, z} consisting of z − αy1 and σ-polynomials resulting from those of Φ by the substitution of αj z for αj+1 y1 (j ∈ N). If η is a generic zero of a component M of M(Φ) such that dim M = 0, then (Y = η, z = α(η1 )) is a generic zero of a component M of Φ . Furthermore, since generic zeros of M and M generate the same extensions of K, dim M = 0 and Eord M = Eord M. It follows that RΦ = RΦ , so one may use Φ ⊆ K{Y, z} instead of Φ ⊆ K{Y }. Continuing such reductions we can replace Φ by a system Ψ of σ-polynomials in some σ-polynomial ring K{Y, z1 , . . . , zp } that satisfies the condition ri ≤ 1

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for i = 1, . . . , s + p. Thus, without loss of generality we can assume that our original system Φ satisfies the condition ri ≤ 1 for i = 1, . . . , s. Before starting the main part of the proof we are going to show that it is sufficient to prove our theorem in the case of inversive difference field K. Indeed, suppose that K is not inversive and let K ∗ be its inversive closure. Let M be a component of M(Φ) with dim M = 0 and let η be a generic zero of M. Then there is a σ-overfield of K ∗ containing all coordinates of η, hence η is a specialization over K ∗ of a generic zero η ∗ of such a component. It follows that η is a specialization of η ∗ over K. Since η is the generic zero of a component of M(Φ), there must be a σ-K-isomorphism of Kη onto Kη ∗  sending η to η ∗ . Clearly, this isomorphism can be extended to a σ-K ∗ -isomorphism of K ∗ η onto K ∗ η ∗ , hence η is a generic zero of a component M∗ of the variety of Φ over K ∗ . Since dim M∗ = 0 and Eord M = Eord M∗ , one can consider M∗ instead of M, so from the very beginning one can assume that the σ-field K is inversive. Thus, from now on we assume that K is inversive, dim M = 0 and ri ≤ 1 (1 ≤ i ≤ s). Then one can easily obtain the inequality Eord M ≤ s. Indeed, let η be a generic zero of M and let K denote the kernel determined by K(η, α(η)) Since Φ ⊆ K[Y, αY ], every realization of K annuls Φ. It is also obvious that η is a regular realization of K. By Theorem 7.4.4, η is a specialization of a principal realization η¯ of K. Since η¯ is a solution of Φ, η is also a principal realization of K, hence δK = 0 and ord M = trdegK K(η) ≤ s. Therefore, Eord M ≤ s. In order to prove the desired inequality Eord M ≤ RΦ we proceed by induction on p where p denotes the number of coordinates of Y which occur only to order 0 in the σ-polynomials of Φ. If p = 0, then RΦ = s, so the statement is true. Suppose that p > 0 and the desired inequality holds for any smaller number. Without loss of generality we can assume that ys is one of the coordinates of Y appearing only to order 0. Let us consider the ring of σ-polynomials K{y1 , . . . , ys−1 , z} (where z is a σ-indeterminate over K{y1 , . . . , ys−1 }), and let Φ∗ denote the family of σpolynomials in this ring obtained by replacing ys by z + αz in the σ-polynomials of Φ. Furthermore, let Y ∗ denote the s-tuple (y1 , . . . , ys−1 , z). Since ys appears in at least one σ-polynomial in Φ (otherwise M(Φ) would not have a zerodimensional component), αz appears in at least one σ-polynomial in Φ∗ . It follows that (7.6.1) RΦ∗ = RΦ + 1 and Y ∗ has p − 1 coordinates which appear only to order 0 in Φ∗ . Let ζ be a generic zero of the prime σ ∗ -ideal {z + αz − ηs } of the ring Lη{z} and let η ∗ denote the s-tuple (η1 , . . . , ηs−1 , ζ). Then Eord Kη ∗ /K = Eord Kη, ζ/K = Eord Kη, ζ/Kη + Eord Kη/K + 1. (7.6.2) ¯ is Let M∗ be a component of M(Φ∗ ) containing η ∗ . If η¯∗ = (¯ η1 , . . . , η¯s−1 , ζ) ∗ ¯ ¯ η1 , . . . , η¯s−1 , ζ + αζ) lies in M(Φ) a generic zero of M , then the s-tuple η¯ = (¯ η  = 0. Since ζ¯ is and specializes to η. It follows that η¯ ∈ M hence σ-trdegK K¯ ∗ η  = 0. σ-algebraic over K¯ η , one has σ-trdegK K¯

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436

Thus, we can apply the inductive hypothesis to obtain the inequality Eord Kη ∗ /K ≤ Eord M∗ ≤ RΦ∗ ,

(7.6.3)

which, together with (7.6.1) and (7.6.2), implies that Eord Kη/K ≤ RΦ. (ii) Proceeding as at the beginning of the proof of part (i) we may assume that Y  = ∅ provided that every component of the modified system Φ∗ is of dimension 0. To show that it is really so, suppose that some component of Φ∗ has positive dimension. Then there exists a solution η ∗ of Φ∗ such that σ-trdegK η
 Kη  , η ∗  > 0 (we use the notation of the proof of (i)). Then σtrdegK Kη  , η ∗  is greater than the number of coordinates of Y  . Since η  , η ∗ is a solution of Φ, this contradicts the assumption that Y  contains a complete set of parameters of every component of M(Φ). Applying similar additional arguments to the corresponding part of the proof of part (i) one can also justify the assumption that the field K is inversive. Thus, from now on we suppose that K is inversive and Y  = ∅. As in the proof of part (i) we proceed with the reduction of Φ to a system of σ-polynomials Φ in a σ-polynomial ring K{z1 , . . . , zt } such that orders of elements of Y  do not exceed 1. Without loss of generality we may assume that the σ-indeterminates z1 , . . . , zt are arranged in such a way that αz1 , . . . , αzr appear in elements of Y  s  while αzr+1 , . . . , αzt do not. Note that r = ri = RΦ = RΦ , and if ri > 0, i=1

then each αj yi (j < ri ) is equated by the newly introduced equations to some s  ri . Distinct αj yi are equated to distinct zk . zk , k ≤ r = i=1

Let M be the component of M(Φ ) corresponding to M, let θ = (θ1 , . . . , θt ) be a generic zero of M , and let K be the kernel with isomorphism τ obtained in the usual way from K(θ, α(θ)). Since every realization of K is a solution of Φ , δK = 0. Furthermore, since θ is a regular realization of K, trdegK(θ) K(θ, α(θ)) = 0, hence ord M = trdegK Kθ = trdegK K(θ). Since Eord M = Eord M = ord M , we obtain that Eord M = trdegK K(θ).

(7.6.4)

We are going to show that trdegK(θ1 ,...,θr ) K(θ) = 0. Indeed, by Corollary 1.6.41 (with the fields K(θ1 , . . . , θr ), K(θ) and K(θ, α(θ1 ), . . . , α(θr )) = K(θ1 , . . . , θt , α(θ1 ), . . . , α(θr )) instead of K, L and M in Corollary 1.6.41, respectively), there exist elements λr+1 , . . . , λt in some overfield of K(θ, α(θ1 ), . . . , α(θr )) with the following properties: (a) The contraction of τ to an isomorphism of the field K(θ1 , . . . , θr ) onto K(α(θ1 ), . . . , α(θr )) extends to an isomorphism ρ of K(θ) into K(θ, λ) where λ denotes (α(θ1 ), . . . , α(θr ), λr+1 , . . . , λt ). (b) ρ(θi ) = λi for i = r + 1, . . . , t. (c) trdegK(θ,α(θ1 ),...,α(θr )) K(θ, λ) = trdegK(θ1 ,...,θr ) K(θ). Let K be the kernel formed by the field K(θ, λ) and the isomorphism ρ. Since Φ is free of αzi for i > r, every realization of K is a solution of Φ .

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Therefore, δK = 0 and 0 = trdegK(θ) K(θ, λ) ≥ trdegK(θ,α(θ1 ),...,α(θr )) K(θ, λ) = trdegK(θ1 ,...,θr ) K(θ). Thus, trdegK(θ1 ,...,θr ) K(θ) = 0 and trdegK K(θ) = trdegK K(θ1 , . . . , θr ).

(7.6.5)

Let the integer h used in the definition of the Greenspan bound be positive and let A be a nonzero σ-polynomial in Φ such that ordyi αh (A) ≤ ri (1 ≤ i ≤ s). Suppose that some transform of some yj appears in A and let k = ordyj A. With A written as a polynomial in αk yj let Ij (A) denote the coefficient of the highest power of αk yj in A. With the above notation we perform the following procedure: if Ij (A) ∈ / Φ(M), we do not change A. Repeating Φ(M), we replace A by Ij (A); if Ij (A) ∈ this process we shall arrive at a nonzero σ-polynomial B ∈ Φ(M) such that (I) B involves only those αj yi that appear in A. (II) There exists p ∈ {1, . . . , s} such that if q = ordyp B, then the coefficient of the highest power of αq yp in B does not belong to Φ(M). Let υ = (υ1 , . . . , υs ) be a generic zero of M. Because of (II) the equations αi (B)(υ) = 0 (0 ≤ i ≤ h − 1) show that the set Υ = {αq (υp ), . . . , αq+h−1 (υp )} is algebraically dependent over K on a subset Σ of the αj (υi ) disjoint from Υ and such that the inclusion αj (υi ) ∈ Σ implies that αj (yi ) appears in one of the σ-polynomials B, α(B), . . . , αh−1 (B) and, therefore (see (I)), in one of A, α(A), . . . , αh−1 (A). Obviously, for any such αj (yi ) we have ri > 0 and j < ri . It follows that these αj (yi ) correspond uniquely to distinct zk , 1 ≤ k ≤ r, as it is shown in the description of the reduction of Φ to Φ . We obtain that the elements of the sets Υ and Σ are in one-to-one correspondence with the elements of disjoint subsets Υ and Σ of {θ1 , . . . , θr } such that every element of Υ is algebraic over K(Σ ). Since Card Υ = h, we have trdegK K(θ1 , . . . , θr ) ≤ r − h = GΦ

(7.6.6)

for every h ≥ 0 (for h = 0 this inequality is obvious). Combining (7.6.4), (7.6.5), and (7.6.6) we obtain the desired inequality Eord M ≤ GΦ.  Remark 7.6.2 With the notation of the last theorem, let ki (1 ≤ i ≤ s) be the smallest integer such that αki yi appears in some σ-polynomial of Φ. Let Ψ be the system obtained from Φ by the substitution of yi for αki yi (i = 1, . . . , s). Then the components of M(Ψ) have the same relative effective orders as the components of Φ. It follows that each ri can be replaced by ri = ri − ki in the bounds in parts (i) and (ii) of Theorem 7.6.1. Exercises 7.6.3 Let K be an ordinary difference field with a basic set σ = {α} and K{y1 , y2 } the algebra of σ-polynomials in two σ-indeterminates y1 and y2 over K. Let A1 , A2 ∈ K{y1 , y2 } and let ordyj Ai = rij (1 ≤ i, j ≤ 2). 1. Show that the Greenspan number for the system {A1 , A2 } is equal to max{r11 + r22 , r12 + r21 }.

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2. Prove that if A1 does not contain transforms of y2 (that is, A1 ∈ K{y1 }) and M is a component of the variety M({A1 , A2 }) with dim M = 0, then Eord M ≤ r11 + r22 . The following three propositions give some estimations of the effective order of a variety in terms of Ritt and Greenspan numbers. Proposition 7.6.4 Let K be an ordinary difference field with a basic set σ = {α} and let K{y1 , . . . , ys } be a ring of σ-polynomials in the set of σ-indeterminates Y = {y1 , . . . , ys } over K. Let Φ be a system of nonzero σ-polynomials in K{Y } and let Y  be a proper subset of Y such that Φ = Φ ∩ K{Y  } = ∅. Furthermore, suppose that every component of the variety M(Φ ) (treated as a variety over K{Y  }) is of dimension 0. Then, for any component M of M(Φ), we have Eord M ≤ GΦ + G(Y  )Φ. PROOF. Let η be a generic zero of M and let η  be a subindexing of η whose coordinates correspond to those of Y  . Let M be a component of M(Φ ) with solution η  . Then Eord Kη  /K = Eord M ≤ GΦ .

(7.6.7)

Let Y  , η  , and Φ denote the complementary indexings or sets to Y  in Y , η  in η, and Φ in Φ, respectively. Let Φ∗ be the set of σ-polynomials in Kη   obtained by substituting η  for Y  in the σ-polynomials of Φ . Then every component of M(Φ∗ ) is of dimension 0. Indeed, if this were not so, then Φ∗ would have a solution η ∗ with σ-trdegK η
 Kη  , η ∗  > 0, and η  , η ∗ would be a solution of Φ, which is impossible. Applying Theorem 7.6.1(ii) we obtain that Eord Kη  , η ∗ /Kη   ≤ GΦ∗ ≤ G(Y  )Φ. 

∗



∗

(7.6.8)



Since Eord M = Eord Kη , η /K = Eord Kη , η /Kη  + Eord Kη  /K, the statement of the proposition follows from the inequalities (7.6.7) and (7.6.8).  The proof of the following statement is similar to the proof of Proposition 7.6.4. Proposition 7.6.5 With the notation of Proposition 7.6.4, let M be a component of the variety M(Φ), and let M be a component of M(Φ ) admitting the solution η  , where η  is the subindexing of a generic zero η of M with the same indices as Y  . Then: (i) If dim M = dim M = 0, then Eord M ≤ RΦ + R(Y  )Φ. (ii) If every component of M(Φ) is of dimension 0, and dim M = 0, then  Eord M ≤ RΦ + G(Y  )Φ. Proposition 7.6.6 With the notation of Proposition 7.6.4 and its proof, suppose that the family Φ consists of a single σ-polynomial A, and every component of M(Φ) is of dimension 0. Then dim M = 0, and therefore Eord M ≤ RΦ + G(Y  )Φ.

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PROOF. Let ζ  be a generic zero of M and let A be the σ-polynomial in Kη  {Y  } obtained by substituting η  for Y  in A. Since A has the solution η  , A cannot be a nonzero element of Kη  . At the same time A = 0 (otherwise, M(Φ) would contain an s-tuple η  , θ where the coordinate θ is σ-transcendental / Kη  . over Kη  ). Therefore, A ∈ ∗ Let A be the σ-polynomial in Kζ  {Y  } obtained by substituting ζ  for Y  in A. Since ζ  specializes to η  over K, the coefficients of A∗ specialize to those / Kζ  . of A . Therefore, A∗ ∈ By Theorem 7.2.1, there is a solution ζ  of A∗ . Then ζ  , ζ  is a solution of Φ, hence σ-trdegK Kζ  , ζ   = 0. It follows that σ-trdegK Kζ  , ζ   = 0, hence  dim M = 0. Theorem 7.6.7 Let K be an ordinary difference field with a basic set σ = {α} and let K{y1 , . . . , ys } be a ring of σ-polynomials in σ-indeterminates y1 , . . . , ys over K. Let Φ = {A1 , . . . , Am } where A1 , . . . , Am are first order σ-polynomials in K{y1 , . . . , ys } (with the notation introduced before Theorem 7.6.1, it means that for every i = 1, . . . , m, rij ≤ 1 (1 ≤ j ≤ s) and at least one rij is equal to 1). If M is an irreducible component of M(Φ) of dimension 0, then Eord M ≤ J (Φ). PROOF. Note, first, that we may assume that K is inversive (to justify this assumption one just needs to repeat the arguments of the first part of the proof of Theorem 7.6.1(i)). Let η¯ = (¯ η1 , . . . , η¯s ) be a generic zero of the variety M. Then η¯ is a principal realization of the kernel K with field K(η, η¯). Indeed, since η¯ is a regular realization of K, it is a specialization of a principal realization η  of K (see Corollary 6.2.16). But η  is a zero of the set {A1 , . . . , Am }; thus η¯ is itself a principal realization. Applying Theorem 5.2.10(i) we obtain that η ). δK = ord M = trdegK K(¯ Since (¯ η , α(¯ η )) is a zero of the ideal (A1 , . . . , Am ) in the polynomial ring (1) K[y1 , . . . , ys , αy1 , . . . , αys ], there is a generic zero (a, a1 ) = (a(1) , . . . , a(s) , a1 , (s) . . . , a1 ) of some component of the (non-difference) variety M (A1 , . . . , Am ) η , α(¯ η )) over K. (We write this as (a, a1 ) −→ such that (a, a1 ) specializes to (¯ K

(¯ η , α(¯ η )). Therefore, p = trdegK K(a) − trdegK K(¯ η ) and q = trdegK K(a1 ) − η )) are nonnegative integers. trdegK K(α(¯ By two applications of Proposition 1.6.66 one obtains s-tuples c and η1 over K such that η )) −→ (¯ η , η1 ) −→ (¯ η , α(¯ η )) (a, a1 ) −→ (c, α(¯ K

K

K

with trdegK K(a, a1 ) − trdegK K(c, α(¯ η )) ≤ q and η )) − trdegK K(¯ η , η1 ) ≤ p. trdegK K(c, α(¯ It follows that η , η1 ) ≤ trdegK K(a1 ) trdegK K(a, a1 ) − trdegK K(¯ −trdegK K(α(¯ η )) + trdegK K(a) − trdegK K(¯ η ).

(7.6.9)

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η ), there is a K-isomorphism of the field K(η1 ) onto Since α(¯ η ) −→ (η1 ) −→ α(¯ K

K

K(α(¯ η )). Furthermore, since K(¯ η , α(¯ η ) forms a kernel, K(¯ η , η1 ) also forms a kernel K. By Theorem 6.2.14, there is a principal realization η of K which specializes to η¯ over K. η , η1 ), η is a zero of the system {A1 , . . . , Am }. Therefore, Since (a, a1 ) −→ (¯ K

the specialization η −→ η¯ is generic and the field K(¯ η , η1 ) is K-isomorphic to K

K(¯ η , α(¯ η )). This isomorphism, together with inequality (7.6.9), implies that η ) = trdegK K(α(¯ η )) ≤ trdegK K(¯ η , α(¯ η )) − trdegK K(¯ η) ord M = trdegK K(¯ +trdegK K(a1 ) − trdegK K(a, a1 ) + trdegK K(a) ≤ 0 + s − trdegK(a) K(a, a1 ). (7.6.10) Let A1 , . . . , Am be the polynomials in the ring K(a)[αy1 , . . . , αys ] obtained by substituting a(1) , . . . , a(s) for y1 , . . . , ys , respectively, in A1 , . . . , Am . Let A =    ) be the m × n-matrix with rij = 1 if αyj appears in Ai , and rij = 0 if not. (rij   Then J (A ) ≤ J (AΦ ) = J (Φ), since rij = 1 only if rij = 1. By Theorem 1.2.47, every component of the variety M (A1 , . . . , Am ) over K(a) has dimension at least s − J (A ), and the last number is greater than or equal to s − J (AΦ ). Furthermore, since a1 is a zero of {A1 , . . . , Am }, there is a generic zero b of a component of M (A1 , . . . , Am ) such that b −−−→ a1 . But K(a)

(a, b) is a zero of Φ, and (a, a1 ) is a generic zero; therefore the specialization is generic and trdegK(a) K(a, a1 ) ≥ s − J (AΦ ). (7.6.11) Combining (7.6.10) and (7.6.11) we obtain that ord M ≤ J (Φ).



A number of results on the Jacobi bound for systems of algebraic differential equations (see, for example, [50], [51] and [110, Section 5.8]) give a hope that the last theorem can be essentially strengthen and generalized to the case of partial difference polynomials.

7.7

Dimension Polynomials and the Strength of a System of Algebraic Difference Equations

Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn }, let K{y1 , . . . , ys }∗ be the ring of inversive difference (σ ∗ -) polynomials in σ ∗ -indeterminates y1 , . . . , ys over K, and let Φ = {fλ | λ ∈ Λ} be a set of σ ∗ -polynomials in in K{y1 , . . . , ys }∗ . As we know, an s-tuple η = (η1 , . . . , ηs ) with coordinates in some σ ∗ -overfield of K is said to be a solution of the system of algebraic difference (σ ∗ -) equations fλ (y1 , . . . , ys ) = 0

(λ ∈ Λ)

(7.7.1)

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441

if Φ is contained in the kernel of the substitution of (η1 , . . . , ηs ) for (y1 , . . . , ys ) which is a difference homomorphism K{y1 , . . . , ys }∗ → Kη1 , . . . , ηs ∗ sending each yi to ηi and leaving elements of K fixed. (Notice that every system of algebraic difference equations in the sense of Definition 2.2.3, that is, a system of the form (7.7.1) with fλ from the ring of σ-polynomials K{y1 , . . . , ys }, can be also viewed as a system of algebraic σ ∗ -equations.) By Theorem 2.5.11, system (7.7.1) is equivalent to some its finite subsystem, that is, there exist finitely many σ ∗ -polynomials f1 , . . . , fp ∈ Φ such that the variety of the set Φ coincides with the variety of the set {f1 , . . . , fp }. Thus, while studying systems of algebraic difference equations in s difference indeterminates over K, one can consider just the systems of the form fi (y1 , . . . , ys ) = 0

(i = 1, . . . , p)

(7.7.2)

where f1 , . . . , fp ∈ K{y1 , . . . , ys }∗ . Definition 7.7.1 A system of algebraic σ ∗ -equations (7.7.2) is called prime if the perfect σ-ideal {f1 , . . . , fp } of the ring K{y1 , . . . , ys }∗ is prime. Since a linear σ ∗ -ideal of the ring K{y1 , . . . , ys }∗ is prime (see Proposition ∗ 2.4.9), every s system of linear homogeneous σ -equations∗ (that is, equations of the form j=1 ωij yj = 0 (0 ≤ i ≤ p) where all ωij are σ -operators over K) is prime. Definition 7.7.2 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let E be the ring of σ ∗ -operators over K. A σ ∗ -operator ω ∈ E is called symmetric if it has the following property. Let ω be represented in the standard form, that is, in the form ω = a1 α1k11 . . . αnk1n + · · · + ar α1kr1 . . . αnkrn where ai ∈ K, ai = 0, and (ki1 , . . . , kin ) = (kj1 , . . . , kjn ) if i = j. If this expression of ω contains a term aα1l1 . . . αnln (a ∈ K, a = 0), then it also contains all terms of the form bα1±l1 . . . αn±ln with nonzero coefficients b ∈ K and all possible distinct combinations of signs before l1 , . . . , ln . (If there are m nonzero numbers among l1 , . . . , ln , then there are 2m such terms.) For example, a σ ∗ -operator a1 α12 α2 + a2 α12 α2−1 + a3 α1−2 α2 + a4 α1−2 α2−1 with nonzero coefficients ai , 1 ≤ i ≤ 4, is symmetric. Lemma 7.7.3 Let u and v be two σ ∗ -operators over an an inversive difference field with a basic set σ = {α1 , . . . , αn }. If at least one of the operators is symmetric, then ord(uv) = ord u + ord v. PROOF. It is easy to see that if ω and ω  are two σ ∗ -operators over K, then of generality we can assume that ord(ωω  ) = ord(ω  ω). Therefore, without loss n u is symmetric. If ord u is equal to the order i=1 |li | of a monomial α1l1 . . . αnln which appears in the standard form of u with a nonzero coefficient, then this form of u contains all monomials α1±l1 . . . αn±ln with nonzero coefficients (the number of such monomials is 2m where m is the number of nonzero entries of

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

the n-tuple (l1 , . . . , ln )). If t = aα1k1 . . . αnkn (a ∈ K, a = 0) is a term in the standard form of v such that ord v = |k1 | + · · · + |kn |, then the standard form of u contains a term t = bα11 l1 . . . αnn ln (b ∈ K, b = 0, and h is 1 or −1 for h = 1, . . . , n) such that the n-tuples (1 l1 , . . . , n ln ) and (k1 , . . . , kn ) belong to the same ortant of Zn . (We refer to decomposition (1.5.5) of Zn into the union of 2n ortants.) It is easy to seethat ord(uv) nis the order of the monomial in the n  product tt , hence ord(uv) = i=1 |ki | + i=1 |li | = ord u + ord v. With the above notation, let Γ denote the free commutative group generated by the set σ. Then the ring of σ ∗ -polynomials R = K{y1 , . . . , ys }∗ coincides with the polynomial ring K[{γyj | γ ∈ Γ, 1 ≤ j ≤ s}], and for any r ∈ N, we can define a polynomial subring Rr = K[{γ(ηj ) | γ ∈ Γ(r), 1 ≤ j ≤ s}] of R (as before, Γ(r) denotes the set {γ ∈ Γ | ord γ ≤ r}). Let us consider a prime system of algebraic difference (σ ∗ -) equations of the form (7.7.2) and let P be a prime difference ideal of R generated (as a perfect difference ideal) by σ ∗ -polynomials in the right-hand sides of the equations of the system. Furthermore, let η i denote the canonical image of ηi in the factor ring R/P (1 ≤ i ≤ s). As in the proof of Theorem 4.6.2, one can easily see that for every r ∈ N, P ∩ Rr is a prime ideal of the ring Rr and the quotient fields of the rings Rr /P ∩ Rr and K[{γ(η j ) | γ ∈ Γ(r), 1 ≤ j ≤ s}] are isomorphic. By Theorem 4.2.5, there exists a numerical polynomial ψP (t) in one variable t such that ψP (t) = trdegK K[{γ(ηj ) | γ ∈ Γ(r), 1 ≤ j ≤ s}] = trdegK (Rr /P ∩ Rr ) for all sufficiently large r ∈ Z, deg ψ(t) ≤ n and the polynomial ψP (t) can be written as 2n aP n t + o(tn ) ψP (t) = n! where aP = σ-trdegK (R/P ). Definition 7.7.4 With the above notation, the numerical polynomial ψP (t) is called the difference dimension polynomial of the prime system of algebraic σ ∗ equations. It follows from Theorem 1.5.11 that the set all of numerical polynomials associated with prime systems of algebraic σ ∗ -equations is well-ordered with respect to the order  (recall that f (t)  g(t) if and only if f (r) ≤ g(r) for all sufficiently large r ∈ Z). Furthermore, by Proposition 4.2.22, if P and Q are two prime σ ∗ -ideals of R such that P ⊇ Q, then ψP (t)  ψQ (t), and the equality ψP (t) = ψQ (t) implies that P = Q. The difference dimension polynomial of a prime system of algebraic difference (σ ∗ -) equations has an interesting interpretation as a measure of strength of a system of such equations in the sense of A. Einstein. In his work [55] A. Einstein defined the strength of a system of partial differential equations governing a physical fields follows: “... the system of equations is to be chosen so that the field quantities are determined as strongly as possible. In order to apply this

7.7. DIMENSION POLYNOMIALS AND THE STRENGTH

443

principle, we propose a method which gives a measure of strength of an equation system. We expand the field variables, in the neighborhood of a point P, into a Taylor series (which presuppposes the analytic character of the field); the coefficients of these series, which are the derivatives of the field variables at P, fall into sets according to the degree of differentiation. In every such degree there appear, for the first time, a set of coefficients which would be free for arbitrary choice if it were not that the field must satisfy a system of differential equations. Through this system of differential equations (and its derivatives with respect to the coordinates) the number of coefficients is restricted, so that in each degree a smaller number of coefficients is left free for arbitrary choice. The set of numbers of “free” coefficients for all degrees of differentiation is then a measure of the “weakness” of the system of equations, and through this, also of its “strength”. ” Considering a system of equations in finite differences over a field of of functions in several real variables, one can use the A. Einstein’s approach to define the concept of strength of such a system as follows. Let Ai (f1 , . . . , fs ) = 0

(i = 1, . . . , p)

(7.7.3)

be a system of equations in finite differences with respect to s unknown grid functions f1 , . . . , fs in n real variables x1 , . . . , xn with coefficients in some functional field K. We also assume that the difference grid, whose nodes form the domain of considered functions, has equal cells of dimension h1 × · · · × hn (h1 , . . . , hn ∈ R) and fills the whole space Rn . As an example, one can consider a field K consisting of a zero function and fractions of the form u/v where u and v are grid functions defined almost everywhere and vanishing at a finite number of nodes. (As usual, we say that a grid function is defined almost everywhere if there are only finitely many nodes where it is not defined.) Let us fix some node P and say that a node Q has order i (with respect to P) if the shortest path from P to Q along the edges of the grid consists of i steps (by a step we mean a path from a node of the grid to a neighbor node along the edge between these two nodes). Say, the orders of the nodes in the two-dimensional case are as follows (a number near a node shows the order of this node). 5 4 3 4

r r r r

4 3 2 3

r r r r

3 2 1 2

r r r r

2 1 P 1

r r r r

3 2 1 2

r r r r

4 3 2 3

r r r r

5 4 3 4

r r r r

r r r r r r r 5 4 3 2 3 4 5 Let us consider the values of the unknown grid functions f1 , . . . , fs at the nodes whose order does not exceed r (r ∈ N). If f1 , . . . , fs should not satisfy any

444

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

system of equations (or any other condition), their values at nodes of any order can be chosen arbitrarily. Because of the system in finite differences (and equations obtained from the equations of the system by transformations of the form fj (x1 , . . . , xs ) → fj (x1 + k1 h1 , . . . , xs + kn hn ) with k1 , . . . , kn ∈ Z, 1 ≤ j ≤ s), the number of independent values of the functions f1 , . . . , fs at the nodes of order ≤ r decreases. This number, which is a function of r, is considered as a “measure of strength” of the system in finite differences (in the sense of A. Einstein). We denote it by Sr . With the above conventions, suppose that the transformations αj of the field of coefficients K defined by αj f (x1 , . . . , xn ) = f (x1 , . . . , xj−1 , xj + hj , . . . , xn ) (1 ≤ j ≤ n) are automorphisms of this field. Then K can be considered as an inversive difference field with the basic set σ = {α1 , . . . , αn }. Furthermore, assume that the replacement of the unknown functions fi by σ ∗ -indeterminates yi (i = 1, . . . , s) in the ring K{y1 , . . . , yn }∗ leads to a a prime system of algebraic σ ∗ -equations (then the original system of equations in finite differences is also called prime). The difference dimension polynomial ψ(t) of the latter system is said to be the difference dimension polynomial of the given system in finite differences. Clearly, ψ(r) = Sr for any r ∈ N, so the difference dimension polynomial of a prime system of equations in finite differences is the measure of strength of such a system in the sense of A. Einstein. By Proposition 2.4.9, every system of homogeneous linear difference equations is prime. Therefore, in order to determine the strength of a linear system of equations in finite differences, one has to find the difference dimension polynomial of the linear σ ∗ -ideal P of K{y1 , . . . , ys }∗ generated by the left-hand sides of the corresponding system of algebraic σ ∗ -equations. This problem, in turn, can be solved either by constructing a characteristic set of the ideal P (using the procedure described before Theorem 2.4.13) and applying the formula in part (iv) of Theorem 4.2.5, or by the method based on Theorem 4.2.9 and formula (4.2.5). The latter approach requires the computation of the difference dimension polynomial of the module of differentials associated with the system. To realize this approach one should consider a generic zero η = (η1 , . . . , ηs ) of the σ ∗ -ideal P , a finitely generated σ ∗ -field extension L = Kη1 , . . . , ηs ∗ of K, and the corresponding E-module of differentials ΩL|K (E denotes the ring of σ ∗ -operators over L). As we have seen, the difference dimension polynomial of the given system of linear difference equations is the difference dimension polynomial of this module associated with the excellent filtration ((ΩL|K )r )r∈Z where (ΩL|K )r is the vector L-space generated by the differentials d(γη) with γ ∈ Γ(r). (As before, Γ denotes the free multiplicative group generated by the basic set σ and Γ(r) = {γ ∈ Γ | ord γ ≤ r}.) In order to find this polynomial, one can construct a free resolution of filtered E-modules of type (4.2.4) and apply formula (4.2.5). Such a resolution has the form dq−1

d

ρ

0 M0 − → ΩL|K → 0 0 → Mq −−−→ Mq−1 → . . . −→

7.7. DIMENSION POLYNOMIALS AND THE STRENGTH

445

and can be constructed from the the following short exact sequences: i

ρ

i

π

0 M0 − → ΩL|K → 0, 0 → Ker ρ −→ 1 0 M1 −→ Ker ρ → 0, 0 → Ker π0 −→

ı

π

2 0 0 → Ker π1 − → M2 −→ Ker π0 → 0,

... where i0 , i1 , . . . are inclusions and π0 , π1 , . . . are natural epimorphisms of filtered E-modules. The following diagram illustrates the construction of the resolution. 0

0 J





JJ ^

Ker π0 π1

J i1  J JJ

^ J ^ d0d2 d13 . . . dM0 M3 M2 M1 i2

  J J π0 i0



J J ^ ^

Ker ρ Ker π1 





J



JJ ^

0

0

0

ρ-

ΩL|K

- 0

J

JJ ^ 0

The next proposition describes the E-module Ker ρ for the system of homogeneous linear difference equations of the form s  ωij yj = 0 (i = 1, . . . , p). (7.7.4) j=1 ∗

(ωij are σ -operators with coefficients in an inversive difference field K with a basic set σ = {α1 , . . . , αn }, and the left-hand sides of the equations are σ ∗ polynomials in the ring K{y1 , . . . , ys }∗ .) Proposition 7.7.5 With the above notation, let η = (η1 , . . . , ηs ) be a generic zero of the linear σ ∗ -ideal P of K{y1 , . . . , ys }∗ generated by the σ ∗ -polynomials s s   ω1j yj , . . . , ωpj yj . Let L = Kη1 , . . . , ηs ∗ , E the ring of σ ∗ -operators j=1

j=1

over L, and ρ : M0 → ΩL|K the natural epimorphism of the free filtered Emodule M0 = Fs0 with free generators f1 , . . . , fs (we use the notation of Exs  dηi (fi → dηi for ample 3.5.4) onto the E-module of differentials ΩL|K = i=1

i = 1, . . . , s). Then Ker ρ is E-submodule of M0 generated by the elements s s   ω1j fj , . . . , gp = ωpj fj . g1 = j=1

j=1

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

446

PROOF. Since d(ax) ⎛ = adx and⎞d(γx) = γdx for any ∈ Γ, x ∈ L, ⎛a ∈ K, γ ⎞ s s s    we obtain that ρ(gi ) = ρ ⎝ ωij fj ⎠ = ωij dηj = d ⎝ ωij ηj ⎠ = d0 = 0 j=1

for i = 1, . . . , p. Therefore,

p 

j=1

j=1

Egi ⊆ Ker ρ.

i=1

In order to show that Ker ρ ⊆

p 

Egi let us introduce, first, the following

i=1

terminology. If an element ω ∈ E is written in the irreducible form ω =

l(ω) 

ak γk

k=1

(l(ω) ∈ N, 0 = ak ∈ L, γk ∈ Γ (1 ≤ k ≤ l(ω)) and γi = γj for i = j), then the q  ωk fk ∈ M0 , then by the number l(ω) will be called the length of ω. If g = k=1 q length of g we mean the number l(g) = k=1 l(ωk ). p  Egi , and let g be an element Suppose that Ker ρ is not contained in of minimal length in Ker ρ \

p 

i=1

Egi . Then g can be written as g =

i=1

where uk =

lk 

uk fk

i=1

cki γki ∈ E (cki ∈ L, γki ∈ Γ). Since ρ(g) =

i=1 lk s  

s 

lk s  

cki γki dηk =

k=1 i=1

cki d(γki ηk ) = 0, the family γη = {γki ηk | 1 ≤ k ≤ s, 1 ≤ i ≤ lk }

k=1 i=1

is algebraically dependent over K (see Proposition 1.7.13). Therefore, there exists a difference polynomial A ∈ K{y1 , . . . , ys }, which is a polynomial in the set of indeterminates {γki yk | 1 ≤ k ≤ s, 1 ≤ i ≤ lk } with coefficients in K, such that A(γη) = 0. Without loss of generality we may assume that A has the minimal total degree among all polynomials in the set of indeterminates {γki yk | 1 ≤ k ≤ s, 1 ≤ i ≤ lk } that vanish at γη. Then there exist k and i (1 ≤ k ≤ s, 1 ≤ i ≤ lk ) such that ∂A ∂A (γη) = |y =η (1≤ν≤s) = 0. ∂(γki yk ) ∂(γki yk ) ν ν ⎡ ⎤ s s   Since A(γη) = 0, A lies in the σ-ideal ⎣ ω1j yj , . . . , ωpj yj ⎦ of the ring j=1

K{y1 , . . . , ys }. Let ωµν =

tµν  r=1

j=1

  bµνr γµνr where tµν ∈ N, bµνr ∈ K, and γµνr ∈Γ

(1 ≤ µ ≤ p, 1 ≤ ν ≤ s, 1 ≤ r ≤ tµν ). Then the σ-polynomial A can be written as

7.7. DIMENSION POLYNOMIALS AND THE STRENGTH

A=

qµ p  

 hµλ γ¯µλ

µ=1 λ=1

=

qµ p  

tµν s  

447

  bµνr γµνr yν

ν=1 r=1

hµλ

µ=1 λ=1

tµν s  

 γ¯µλ (bµνr )¯ γµλ γµνr yν ,

(7.7.5)

ν=1 r=1

where qµ ∈ N, hµλ ∈ K{y1 , . . . , ys }, γ¯µλ ∈ Γ (1 ≤ µ ≤ p, 1 ≤ λ ≤ qµ ).  ya | 1 ≤ a ≤ s, 1 ≤ b ≤ ma for some positive integers Let Φ = {γab m1 , . . . , ms } be the set of all terms γyi (γ ∈ Γ, 1 ≤ i ≤ s) which appear in A or in the last part of equation (7.7.5). (In particular, γki yk ∈ Φ for any k = 1, . . . , s; i = 1, . . . , lk such that γki yk appears in A.) ∂A Let Aab =  y ) (γη) for a = 1, . . . , s; b = 1, . . . , ma (it is easy to see that ∂(γab a  if γab ya is not equal to one of the γki yk (1 ≤ k ≤ s, 1 ≤ i ≤ lk ), then Aab = 0). The equality A(γη) = 0 implies dA(γη) = 0, that is, lk s   k=1 i=1

k a   ∂A  (γη)d(γki yk ) = Aki γki dηk = Aab γab dηa = 0. ∂(γki yk ) a=1 i=1

s

l

s

k=1

m

b=1

Let us show that g0 =

ma s  

 Aab γab fa =

a=1 b=1

lk s  

Aki γki fk ∈

k=1 i=1

p 

Egi .

i=1

Indeed, by (7.7.5) we have ⎫ ⎧ ⎡ ⎤ qµ tµν ma ⎨ s  s  s  ⎬    ∂   ⎣ ⎦ (γη) γab g0 = h γ ¯ (b )¯ γ γ y fa µλ µλ µνj µλ ν µνj  y ) ⎭ ⎩ ∂(γab a a=1 µ=1 ν=1 j=1 b=1

=

λ=1

 qµ p  ma  s   a=1 b=1 µ=1 λ=1



⎛

⎞

tµν s  

∂  ¯µλ ⎝ bµνj γµνj ην ⎠  y ) hµλ γ ∂(γab a ν=1 j=1

  ∂(¯ γµλ γµνj yν )  + hµλ (γη) (γη) γab γ¯µλ (bµνj ) fa  y ) ∂(γ a ab ν=1 j=1 tµν s  

=

qµ tµν p  s   

hµλ (γη)¯ γµλ (bµνj )

µ=1 λ=1 ν=1 j=1

=

qµ tµν p  s   

ma s    ∂(¯ γµλ γµνj yν ) a=1 b=1

 y ) ∂(γab a

 (γη)γab fa

 hµλ (γη)¯ γµλ (bµνj )¯ γµλ γµνj fν

µ=1 λ=1 ν=1 j=1

=

qµ p   µ=1 λ=1

hµλ (γη)¯ γµλ

s  ν=1

ωµν fν =

qµ p   µ=1 λ=1

hµλ (γη)¯ γµλ gµ ∈

p  i=1

Egi .

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

448

As we have seen, not all Aki (1 ≤ k ≤ s, 1 ≤ i ≤ lk ) are equal to zero. Without p  loss of generality one can assume that A11 = 0. Since g ∈ Ker ρ\ Egi and g0 ∈ p 

i=1

Egi , we have g − c11 (A11 )−1 g0 ∈ Ker ρ \

p i=1

Egi and l(g − c11 (A11 )−1 g0 ) <

i=1

l(g) (since the irreducible form of g0 contains only those elements of the form γfi (γ ∈ Γ, 1 ≤ i ≤ s) which appear in g, and g − c11 (A11 )−1 g0 contains fewer such elements than g). This contradiction with the choice of g shows that p  Egi .  Ker ρ = i=1

Example 7.7.6 (Linear homogeneous symmetric σ ∗ -equation) Let K be an inversive difference field of zero characteristic with a basic set σ = {α1 , . . . , αn } and let E denote the ring of σ ∗ -operators over K. Consider a difference (σ ∗ -) equation ω1 y1 + · · · + ω s ys = 0

(7.7.6)

where y1 , . . . , ys are σ ∗ -indeterminates over K and ω1 , . . . , ωs ∈ E are symmetric E-operators (see Definition 7.7.2). Let P denote the linear (and therefore prime) reflexive difference ideal generated by the σ ∗ -polynomial ω1 y1 + · · · + ωs ys and let L denote the inversive difference field of quotients of K{y1 , . . . , ys }∗ /P . After setting ηi = yi + P ∈ L (1 ≤ i ≤ s) one may consider (7.7.6) as a defining σ ∗ equation on the system of σ ∗ -generators η = (η1 , . . . , ηs ) of the σ ∗ -field extension L/K (in the sense that the left-hand side of the equation generates the defining σ ∗ -ideal of L/K). Let EL denote the ring of σ ∗ -operators over L. As we have seen, elements dη1 , . . . , dηs (where dηi denotes dL|K ηi ) generate the module of differentials ΩL|K over the ring EL , so one can consider a natural homomorphism ρ of a free filtered EL -module Fs0 with free generators f1 , . . . , fs onto ΩL|K (ρ : fi → dηi , s  ωi fi . 1 ≤ i ≤ s). By Proposition 7.7.5, one has Ker ρ = EL g where g = i=1

The induced filtration on Ker ρ (with respect to which the embedding 0 of filtered EL -modules) is of the form Ker  ρ → Fss is a homomorphism  # Ker ρ (EL )r fi . By Theorem 3.5.7, this filtration is excellent. i=1

r∈Z

We are going to show that for all sufficiently large r ∈ Z, Ker ρ

s #

(EL )r fi = (EL )r−k g

(7.7.7)

i=1

where k = max1≤i≤s {ord ωi }. Indeed, since the σ ∗ -operators ω1 , . . . , ωs are symmetric, ord(uωi ) = ord u+ord ωi ≤ (r−k)+k = r for any u ∈ (EL )r−k , 1 ≤ i ≤ s (see Lemma 7.7.3).

7.7. DIMENSION POLYNOMIALS AND THE STRENGTH  s 

Therefore, (EL )r−k g = Conversely, let A ∈ Ker ρ



 uωi fi | u ∈ (EL )r−k

i=1 s 

⊆ Ker ρ

449 s 

(EL )r fi .

i=1

(EL )r fi Then

i=1

A=

s 

vi fi = ωg =

i=1

s 

ωωi fi

i=1

for some vi ∈ (EL )r (i = 1, . . . , s) and ω ∈ EL . Since {f1 , . . . , fs } is basis of the free EL -module Fs0 , the last equality implies that vi = ωωi for i = 1, . . . , s. Since the σ ∗ -operators ωi are symmetric, one can apply Lemma 7.7.3 and obtain that ord ω = ord vi − ord ωi (1 ≤ i ≤ s). It follows that there exists j ∈ {1, . . . , s} such that ord ω = ord vj − k ≤ r − k, so that ω ∈ (EL )r−k . Therefore, s  (EL )r fi ⊆ (EL )r−k g, so that equality (7.7.7) is proved. Ker ρ i=1

Let us consider the mapping π : F1k → Ker ρ such that π(λh) = λg for every λ ∈ EL (h denotes the element of the basis of F1k ). It follows from (7.7.7) that π is a homomorphism of filtered EL -modules and the sequence of such modules β π F1k − → Ker ρ − → Fs0 , where β is the injection, is exact in Ker ρ. Furthermore, it is easy to see that Ker ρ = 0. (Indeed, if λh ∈ Ker π (λ ∈ EL ), then λg = s  λωi fi = 0. It follows that λ1 ω1 = · · · = λs ωs = 0 whence λ = 0.) Thus, we i=1

obtain the following free resolution of the module ΩL|K : 0 Fs0 − → ΩL|K → 0. 0 → F1k −→

d

π

(7.7.8)

where d0 = β ◦ π. Applying to this resolution formulas (4.2.5) and (3.5.5), we obtain the dimension polynomial Ψη|K (t) of σ ∗ -equation (7.7.6): Ψη|K (t) = s

n 

(−1)

i=0

  



 n t−k+i n t+i n−i i n − . 2 (−1) 2 i i i i i=0 (7.7.9)

n−i i

The last formula allows one to find the invariants σ ∗ -trdegK L, σ ∗ -typeK L, and σ ∗ -t.trdegK L of the σ ∗ -field extension L/K. These invariants coincide, respectively, withthe characteristics σ ∗ -dim P , σ ∗ -type P , and σ ∗ -t.dim P of the  ∗ s  ωi yi : if s > 1, then σ ∗ -typeK L = σ ∗ -type P = n and σ ∗ -ideal P = i=1

σ ∗ -trdegK L = σ ∗ -t.trdegK L = σ ∗ -dim P = σ ∗ -t.dim P = s − 1; if s = 1 and k ≥ 1 (the case k = 0 is trivial), then σ ∗ -typeK L = σ ∗ -type P = n − 1, σ ∗ trdegK L = σ ∗ -dim P = 0, and σ ∗ -t.trdegK L = σ ∗ -t.dim P = 2k. Let K be a field of functions of n real variables x1 , . . . , xn , let h1 , . . . , hn be some real numbers, and let the mappings αi : K → K defined by the equality (αi f )(x1 , . . . , xn ) = f (x1 , . . . , xi−1 , xi + hi , xi+1 , . . . , xn ) (f (x1 , . . . , xn ) ∈

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

450

K, 1 ≤ i ≤ n) be automorphisms of the field K. Then K can be treated as an inversive difference field with the basic set σ = {α1 , . . . , αn } and the finite difference approximation of linear differential equations produces algebraic σ ∗ equations of the form (7.7.6). In what follows we determine dimension polynomials of such σ ∗ -equations associated with some classical differential equations. Example 7.7.7 Using the five-point scheme for the finite difference approx∂2y ∂2y imation of the two-dimension Laplace equation + = 0 we obtain a 2 ∂x1 ∂x22 σ ∗ -equation of the form   1 1 −1 −1 (α − α − 2) + (α − α − 2) y=0 1 2 1 2 h21 h22 where 0 = hl , h2 ∈ R. Applying (7.7.9) (with n = Card σ = 2), we see that the dimension polynomial for this equation is Ψ(t) =

2 

(−1)2−i

i=0

 

  t+i−1 2 i t+i = 4t. 2 − i i i

Note, that dimension polynomials of some other algebraic σ ∗ -equations which result from the finite difference approximations of classic differential equations have the same form. For example, the dimension polynomial of the σ ∗ -equation   1 δ −1 −1 −1 (α1 − α1 ) − (α2 − α1 − α1 − α2 ) y = 0 (h1 , h2 , δ ∈ R) 2h1 h2 which results from the finite difference approximation of the simple diffusion equation by the scheme of Dufort and Frankel (see [160, Sect. 8.2, Table 8.1]), is also equal to 4t. Example 7.7.8 The algebraic σ ∗ -equation obtained by the finite difference approximation of two-dimensional Laplace equation via the nine-point scheme (see [77, Sect. 5.1]), has the form  h2 +h2 1 (α1 +α1−1 − 2)+ h12 (α2 +α2−1 − 2)+ 112 2 (α1 +α1−1 − 2)(α2 +α2−1 −2) y = 0 h2 1

2

(h1 , h2 , δ ∈ R). Applying formula (7.7.9) we find the dimension polynomial Ψ(t) for this equation (here, as in the previous case, n = Card σ = 2): Ψ(t) =

2  i=0

 

  t+i−2 t+i 2 = 8t − 4. − 2 i i i

2−i i

(−1)

Example 7.7.9 Consider the algebraic σ ∗ -equation obtained by finite difference approximation of the Lorentz equation for the potentials of electromagnetic

7.7. DIMENSION POLYNOMIALS AND THE STRENGTH

451

field (see [174, App. 2 to Ch. 5, Sect. 2]) if every partial derivative is replaced by the corresponding central difference. This equation has the form 3  1 1 (αi − αi−1 )yi + (α4 − α4−1 )y4 = 0 h τ i=1 i

where h1 , h2 , h3 , τ ∈ R. The dimension polynomial of the equation is Ψ(t) = 4

4  i=0

(−1)4−i 2i

  



 4 4 t+i 4 t+i−1 − (−1)4−i 2i i i i i i=0

 



t+2 t+3 t+4 −5. +40 −80 = 48 2 3 4 The two following examples deal with algebraic σ ∗ -equations obtained from the well-known systems of differential equations (see, for example, [150, Appendix, Sect. A4 and Sect. A15]) by a finite difference approximation where each partial derivative is replaced with the corresponding central difference. In both of these examples K denotes an inversive difference field of coefficients, and αi (1 ≤ i ≤ 4) are the elements of the basic set σ of K. The systems of algebraic σ ∗ -equations are treated as defining systems of equations on the generators η1 , . . . , ηs of the σ ∗ -extension L = Kη1 , . . . , ηs ∗ . The ring of σ ∗ -operators over L is denoted by E. In each example we write a free resolution of the module of differentials ΩL|K , compute the dimension polynomial Ψ(t) of the corresponding system, and find the invariants σ ∗ -trdeguK L, σ ∗ -typeK L, and σ ∗ -t.trdegK L. Example 7.7.10 The finite difference approximation of the Dirac equation (with zero mass) produces the following system of linear σ ∗ -equations: ⎧ a4 (α4 − α4−1 )y1 − a3 (α3 − α3−1 )y3 − [a1 (α1 − α1−1 ) + a2 (α2 − α2−1 )]y4 = 0, ⎪ ⎪ ⎪ ⎨a (α − α−1 )y − [a (α − α−1 ) − a (α − α−1 )]y + a (α − α−1 )y = 0, 4 4 2 1 1 2 2 3 3 3 4 4 1 2 3 −1 −1 −1 −1 ⎪ (α − α )y + [a (α − α ) + a (α − α )]y − a (α − α )y a 3 3 1 1 1 2 2 2 4 4 3 = 0, ⎪ 3 1 2 4 ⎪ ⎩ −1 −1 −1 −1 [a1 (α1 − α1 ) − a2 (α2 − α2 )]y1 − a3 (α3 − α3 )y2 − a4 (α4 − α4 )y4 = 0 where the coefficients ai (1 ≤ i ≤ 4) belong to the field of constants C(K) of the σ ∗ -field K. Let ρ : F40 → ΩL|K be the natural epimorphism of the free filtered E-module F40 with free generators f1 , f2 , f3 , f4 onto ΩL|K (in our case 4 L = K(η1 , η2 , η3 , η4 )). By Proposition 7.7.5, we have Ker ρ = i=1 Egi where g1 = a4 (α4 − α4−1 )f1 − a3 (α3 − α3−1 )f3 − [a1 (α1 − α1−1 ) + a2 (α2 − α2−1 )]f4 , g2 = a4 (α4 − α4−1 )f2 − [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )]f3 + a3 (α3 − α3−1 )f4 , g3 = a3 (α3 − α3−1 )f1 + [a1 (α1 − α1−1 ) + a2 (α2 − α2−1 )]f2 − a4 (α4 − α4−1 )f3 , g4 = [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )]f1 − a3 (α3 − α3−1 )f2 − a4 (α4 − α4−1 )f4 .

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Let us show that Ker ρ ∩

4 

Er fi =

i=1

4 

Er−1 gi .

(7.7.10)

i=1

4 Indeed, the inclusion i=1 Er−1 gi ⊆ Ker ρ is obvious (since σ ∗ -operator coef4 ficients in the expressions of gi (1 ≤ i ≤ 4) are symmetric, ωgj ∈ i=1 Er fi 4 4 for any ω ∈ Er−1 ). Conversely, let H = i=1 ωi fi ∈ Ker ρ ∩ i=1 Er fi where 4 ωi ∈ Er (1 ≤ i ≤ 4). Then there exist λ1 , λ2 , λ3 , λ4 ∈ E such that i=1 ωi fi = 4 j=1 λj gj . Comparing the coefficients of fi 1 ≤ i ≤ 4) in the left- and righthand sides of the last equality, we obtain a system of four equations which, after reducing to the row-echelon form, can be written as follows: λ1 a4 (α4 − α4−1 ) + λ3 a3 (α3 − α3−1 ) + λ4 [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )] = ω1 , λ2 a4 (α4 − α4−1 ) + λ3 [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )] − λ4 a3 (α3 − α3−1 ) = ω2 , λ3 u = ω3 , λ4 u = ω4

(7.7.11)

where u = a22 (α2 − α2−1 )2 + a24 (α4 − α4−1 )2 − a21 (α1 − α1−1 ) − a23 (α3 − α3−1 ), ω3 = ω3 a4 (α4 − α4−1 ) − ω1 a3 (α3 − α3−1 ) − ω2 [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )], ω4 = ω4 a4 (α4 − α4−1 ) − ω1 [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )] + ω2 a3 (α3 − α3−1 ). Since the σ ∗ -operator u is symmetric and ω3 , ω4 ∈ Er+1 , one has ord λ3 = ord ω3 − ord u ≤ r − 1 and similarly ord λ4 ≤ r − 1. These inequalities, together with the first two equations of (7.7.11), imply that ord(λ1 a4 (α4 − α4−1 )) ≤ r and 4 ord(λ2 a4 (α4 − α4−1 )) ≤ r, hence λ1 , λ2 ∈ Er−1 . It follows that H ∈ i=1 Er−1 gi , so the equality (7.7.10) is proved. " ! 4 = Let us consider the mapping π : F41 → Ker ρ such that π i=1 ui hi 4 1 i=1 ui gi where {h1 , h2 , h3 , h4 } is a basis of the free filtered module F4 over E and u1 , u2 , u3 , u4 ∈ E). Then equality (7.7.10) implies that the sequence of β◦π

ρ

filtered E-modules F41 −−−→ F40 − → ΩL|K → 0, where β is the injection Ker ρ → F40 , is exact in F40 . 4 4 If Q = i=1 µi hi ∈ Ker π for some µ1 , µ2 , µ3 , µ4 ∈ E, then i=1 µi gi = 0. After replacing every gi (1 ≤ i ≤ 4) with its expression in terms of f1 , . . . , f4 and equating the coefficients of every fj to zero we obtain that the σ ∗ -operators µj (1 ≤ j ≤ 4) satisfy a system of equations which, after reducing to row-echelon form, has the form (7.7.11) with ω1 = ω2 = ω3 = ω4 = 0 (and with µi = λi , 1 ≤ i ≤ 4). Since such a system has the unique solution µ1 = · · · = µ4 = 0, we obtain, that Q = 0, so that Ker π = 0. Thus, the free resolution of the module ΩL|K has the form ρ

0 F40 − → ΩL|K → 0. 0 → F41 −→

d

7.7. DIMENSION POLYNOMIALS AND THE STRENGTH

453

where d0 = β ◦ π. Applying formula (4.2.5) we obtain that Ψ(t) = 4

4  i=1

(−1)4−i 2i

 

  t+i−1 t+i 4 − i i i

 



t+1 t+2 t+3 − 32. + 96 − 128 = 64 1 2 3 In this case σ ∗ -trdegK L = 0, σ ∗ -typeK L = 3, σ ∗ -t.trdegK L = 8. Exercise 7.7.11 Let K be an inversive difference field with a basic set σ = {α1 , α2 , α3 , α4 } and let K{y1 , y2 , y3 , y4 }∗ be the ring of σ ∗ -polynomials in σ ∗ indeterminates y1 , y2 , y3 , y4 over K. Let us consider the system of σ ∗ -equations from Example 7.7.10 and denote by P the linear σ ∗ -ideal of K{y1 , y2 , y3 , y4 }∗ generated by the σ ∗ -polynomials A1 = a4 (α4 − α4−1 )y1 − a3 (α3 − α3−1 )y3 − [a1 (α1 − α1−1 ) + a2 (α2 − α2−1 )]y4 , A2 = a4 (α4 − α4−1 )y2 − [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )]y3 + a3 (α3 − α3−1 )y4 , A3 = a3 (α3 − α3−1 )y1 + [a1 (α1 − α1−1 ) + a2 (α2 − α2−1 )]y2 − a4 (α4 − α4−1 )y3 , A4 = [a1 (α1 − α1−1 ) − a2 (α2 − α2−1 )]y1 − a3 (α3 − α3−1 )y2 − a4 (α4 − α4−1 )y4 in the left-hand side of this system. a) Show, that the σ ∗ -ideal P has a characteristic set consisting of the following eighteen σ ∗ -polynomials: A1 , A2 , A3 , A4 , A5 = [−a21 (α1 − α1−1 )2 + a22 (α2 − α2−1 )2 − a23 (α3 − α3−1 )2 + a24 (α4 − α4−1 )2 ]y1 , A6 = [−a21 (α1 − α1−1 )2 + a22 (α2 − α2−1 )2 − a23 (α3 − α3−1 )2 + a24 (α4 − α4−1 )2 ]y2 , A7 = [−a3 (α3 − α3−1 )A1 + [a1 (α1 − α1−1 ) + a2 (α2 − α2−1 )]A2 , A8 = α1−1 A1 , A9 = α1−1 α2−1 A1 , A10 = α3−1 A2 , A11 = α4−1 A3 , A12 = α4−1 A4 , A13 = α1−1 A5 , A14 = α1−1 α2−1 A5 , A15 = α1−1 A6 , A16 = α1−1 α2−1 A6 , A17 = α1−1 A7 , A18 = α1−1 α2−2 A7 . Consider the leaders of the σ ∗ -polynomials A1 , . . . , A18 and use Theorems 4.2.5 and 1.5.14 to find the the dimension polynomial Ψ(t) of the linear σ ∗ ideal P . Example 7.7.12 The system of linear σ ∗ -equations obtained by the finite difference approximation of the system of Lame equations has the form   1 1 2 −2 (α1 α2 − α1 α2−1 + α1−1 α2−1 )y2 Λ − 2 (α1 + α1 − 2) y1 − h1 h1 h2 1 (α1 α3 − α1 α3−1 + α1−1 α3−1 )y3 = 0, h1 h3   1 2 1 −1 −1 −1 −2 (α1 α2 − α1 α2 + α1 α2 )y1 + Λ − 2 (α1 + α1 − 2) y2 − h1 h2 h1 −

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

454

1 (α2 α3 − α2 α3−1 + α2−1 α3−1 )y3 = 0, h2 h3 1 1 − (α1 α3 − α1 α3−1 + α1−1 α3−1 )y1 − (α2 α3 − α2 α3−1 + α2−1 α3−1 )y2 h1 h3 h2 h3   1 + Λ − 2 (α32 + α3−2 − 2) y3 = 0 (7.7.12) h3

−

4  1 2 1 2 −2 (α4 + α4−2 − 2) − a 2 2 (αi + αi − 2) (a and hi (1 ≤ i ≤ 4) h4 h 4 i=1 are constants of the σ ∗ -field K).

where Λ = −

Proceeding as in the previous example, we obtain that the free resolution of the module of differentials in this case has the form 0 0 → F32 −→ F30 − → ΩL|K → 0.

d

π

Using this resolution we arrive at the following expression for the dimension polynomial of system (7.7.12):  

  t+i−2 t+i 4 − (−1) 2 Ψ(t) = 3 i i i i=1  



t+1 t+2 t+3 − 120. + 240 − 240 = 96 1 2 3 4 

4−i i

The invariants of the σ ∗ -field extension L/K defined by our system are as follows: σ ∗ -trdegK L = 0, σ ∗ -typeK L = 3, and σ ∗ -t.trdegK L = 12. The natural generalization of the concept of the strength of a system of difference equations arises in the case when one evaluates the maximal number of values of unknown grid functions that can be chosen arbitrarily in a region which is not symmetric with respect to a fixed node P. More precisely, using the previous settings (where algebraic σ ∗ -equations are considered over an inversive difference field with a basic set σ = {α1 , . . . , αn }), let us denote the automorphisms αi−1 by αn+i (1 ≤ i ≤ n), add to the system (7.7.2) ns new equations ¯ = {α1 , . . . , α2n } (αi αn+i − 1)fj = 0 (1 ≤ i ≤ n, 1 ≤ j ≤ s) and divide the set σ ¯p . Then for any r1 , . . . , rp ∈ N, the transcendence into p disjoint subsets σ ¯1 , . . . , σ degree of the field    k1 k2n ki ≤ ri for i = 1, . . . , p K α1 . . . α2n fj | k1 , . . . , k2n ∈ N, 1 ≤ j ≤ s, ν∈¯ σi

over K is a function of r1 , . . . , rp which can be naturally treated as a generalized strength of the given system of equations in finite differences. Theorem 4.2.16 shows that this characteristic is a polynomial function of r1 , . . . , rp . The computation of the corresponding difference dimension polynomial in p variables can be performed with the use of the technique of characteristic sets with respect to

7.8. DIMENSION POLYNOMIALS FOR TWO TRANSLATIONS

455

several orderings developed in section 4.2 (see the proof of Theorems 4.2.16 and 4.2.17). However, this process is quite lengthy even for simple nonlinear difference equations. In the case of linear difference equations one can use the obvious analogue of Theorem 4.2.9 for multidimensional filtrations and Proposition 7.7.5 ¯p )-dimension polyto reduce the problem to the computation of the (¯ σ1 , . . . , σ nomial of a finitely generated σ ¯ -K-module with the basic set σ ¯ = {α1 , . . . , α2n } ¯p of this basic set. and the corresponding fixed partition σ ¯=σ ¯1 ∪ · · · ∪ σ Example 7.7.13 Let us consider the algebraic difference equation α1 y1 − α2 y2 = 0

(7.7.13)

over an inversive difference field with a basic set σ = {α1 , α2 }. (The σ ∗ indeterminates y1 and y2 represent unknown grid functions.) Using the language of the A. Einstein’s approach, let us determine the number S(r1 , r2 ) of values of the grid functions that can be chosen freely (that is, algebraically independently) in the nodes with coordinates (a, b) such that |a| ≤ r1 , |b| ≤ r2 . (We fix the origin at some node P of the grid and assign two coordinates to every node of our two-dimensional grid in the natural way.) By Theorem 4.2.17, there exists a polynomial Ψ(t1 , t2 ) in two variables t1 and t2 such that S(r1 , r2 ) = Ψ(r1 , r2 ) for all sufficiently large (r1 , r2 ) ∈ Z2 . Using the remark before this example, one can find Ψ(r1 , r2 ) as the (σ1 , σ2 )∗ dimension polynomial of the σ ∗ -K-module M with two generators h1 and h2 and one defining relation α1 h1 − α2 h2 = 0. Using the result of Example 3.5.41, where this polynomial was computed with the use of a generalized Gr¨obner basis method developed in section 3.3, we obtain that Ψ(t1 , t2 ) = 4t1 t2 + 4t1 + 4t2 + 2. It follows from Theorem 4.2.17 that the degree 2 of the polynomial Ψ(t1 , t2 ) and the coefficient 4 of the term t1 t2 are the invariants of the inversive difference field extension determined by equation (7.7.13).

7.8

Computation of Difference Dimension Polynomials in the Case of Two Translations

Let K be an inversive difference field of zero characteristic with a basic set σ = {α, β}, Γ a free commutative group generated by elements α and β, and for every k ∈ N let Γ(k) = {γ = αi β j ∈ Γ | ord γ = |i| + |j| ≤ k}. Furthermore, let K{y}∗ be the ring of σ ∗ -polynomials in one σ ∗ -indeterminate y over K, and for every σ ∗ -polynomial A ∈ K{y}∗ let τ (A) denote the set of all points (p, q) of the real plane R2 such that the term αp β q y appears in A. Finally,

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

let πA denote the closed rectangle of the smallest area in the set of all closed rectangles which contain τ (A) and whose sides are perpendicular to the vectors 1, 1 and 1, −1. The following example illustrates the introduced concepts. Example 7.8.1 Let us consider the σ ∗ -polynomial A = (αβ −1 y)(α−1 βy)2 + (αy)(β −1 y) + 2α2 y + 2β 2 y − β −1 y + y 2 . Then the set τ (A) consists of the points (1, −1), (−1, 1), (1, 0), (0, −1), (2, 0), (0, 2), and (0, 0). In this case the rectangle πA is as follows. 6 M2 • (0, 2) @ @ @ @ @ (−1, 1) • @ @ @ @ M3 @•M1 • • @ (0, 0) (1, 0) (2, 0) @ @ @ •(0, −1) • (1, −1) @ @ @ @ @ @ M 4

Exercise 7.8.2 Let M (x1 , x2 ) and M  (x1 , x2 ) be two adjacent vertices of the rectangle πA (A ∈ K{y}). Prove that |x1 − x1 | + |x2 − x2 | ∈ N. Exercise 7.8.3 Let A be a σ ∗ -polynomial in K{y} and let d be a real number such that |p| + |q| ≤ d for every term αp β q which appears in A. Prove that |x1 | + |x2 | ≤ d for any point M (x1 , x2 ) of the rectangle πA . The following definition generalizes the concept of effective order of an ordinary difference polynomial introduced in Section 2.4. Definition 7.8.4 Let ρ denote a metric on R2 such that ρ(M, M  ) = |x1 − x1 | + |x2 − x2 | for any two points M (x1 , x2 ), M  (x1 , x2 ) ∈ R2 . With the above notation, the rectangle πA of a σ ∗ -polynomial A ∈ K{y} is called the basic rectangle of A; the perimeter of this rectangle in the metric ρ is called the σ ∗ order of the σ ∗ -polynomial A and is denoted by σ ∗ -ord A.

7.8. DIMENSION POLYNOMIALS FOR TWO TRANSLATIONS

457

It is easy to see that if A is the σ ∗ -polynomial considered in Example 7.8.1, then σ ∗ -ord A = 16. Lemma 7.8.5 With the above notation, σ ∗ -ord A = σ ∗ -ord γA for any σ ∗ polynomial A ∈ K{y}∗ and for any γ ∈ Γ. PROOF. Let γ = αp β q where p, q ∈ Z. Then the basic rectangle πγA is the result of shifting of the rectangle πA by the vector p, q. Obviously this geometric transformation does not change the perimeter of the rectangle.  Theorem 7.8.6 Let K be an inversive difference field with a basic set σ = {α, β} and let K{y}∗ be the ring of σ ∗ -polynomials of one σ ∗ -indeterminate / K, and let y over K. Let A be a linear σ ∗ -polynomial in the ring K{y}∗ , A ∈ P = [A]∗ be the σ ∗ -ideal of the ring K{y}∗ generated by A. Then σ ∗ -type P = 1, σ ∗ -t.dim P = (σ ∗ -ord A)/4, and the dimension polynomial ΨP (t) of the linear σ ∗ -ord A t + a0 , where a0 ∈ Z. σ ∗ -ideal P has the form ΨP (t) = 2 PROOF. By Corollary 2.4.12, one can choose a characteristic set A of the ideal P as the set of minimal elements of the set {γA | γ ∈ Γ} with respect to the preorder  on the ring K{y}∗ considered in Corollary 2.4.12. Let us 4  2 2 ¯ − × N, Zj where Z1 = N × N, Z2 = Z represent Z in the form Z = j=1

¯− × Z ¯ − , and Z4 = N × Z ¯ − (we use the notation of section 1.5). Setting Z3 = Z Γj = {γ = αp β q | (p, q) ∈ Zj } and Yj = {γy | γ ∈ Zj } (1 ≤ j ≤ 4) we obtain the 4 4   Γj and Y = Yj of the group Γ and corresponding representations Γ = j=1

j=1

the set of terms Y = {γy | γ ∈ Γ}, respectively. Since P = [γ(A)]∗ for any γ ∈ Γ, without loss of generality we may assume that A belongs to the described characteristic set of the σ ∗ -ideal P and has the following properties: (i) The leader uA belongs to Y1 . (ii) The sides of the basic rectangle πA , which are parallel to the vector 1, 1, are not shorter than the other sides of the rectangle (with respect to the metric ρ). (iii) The rectangle πA contains the origin (0, 0). Let us denote the sides of the basic rectangle by l1 , . . . , l4 where the numeration goes in the counterclockwise direction starting with the side l1 which is parallel to the vector −1, 1 and intersects the first quadrant. Furthermore, let di denote the distance (in metric ρ) from the origin to the side li (1 ≤ i ≤ 4). 4 It is easy to see that σ ∗ -ord A = i=1 di and if uA = αi β j y, then i + j = d1 . For every γ = αp β q ∈ Γ, the set τ (γA) is obtained by the shift of the set τ (A) by the vector p, q. Therefore, in order to determine a characteristic set of the ideal [A]∗ we can consider only points of the set τ (A) which lie on

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CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

the sides li (1 ≤ i ≤ 4) of the basic rectangle πA . Actually, all leaders of σ ∗ polynomials γ(A) (γ ∈ Γ), which lie in the same quadrant, are defined by one of the monomials of the σ ∗ -polynomial A: the leaders uγA from the jth quadrant (1 ≤ j ≤ 4) can be obtained by multiplication of certain term vj in the σ ∗ polynomial A by some elements γ  ∈ Γ. Clearly, v1 = uA and if γ ∈ Γ1 , then / A for every γ ∈ Γ1 , γ = 1. uγA = γv1 . It follows that γA ∈ In order to find elements of the characteristic set A, whose leaders lie in the second quadrant, we will consider polynomials γA with γ ∈ Γ2 . Let  be an order on Γ such that αi β j  αp β q if and only if the 3-tuple (|i| + |j|, i, j) is less than (|p| + |q|, p, q) with respect to the lexicographic order on Z3 . Furthermore, for every set Λ ⊆ Γ, let µ(Λ) denote the minimal element of the set Λ with respect to the order . Consider a sequence Λ0 , Λ1 , Λ2 , . . . of subsets of Γ such that Λ0 = Γ2 , $ ∅ if uµ(Λ2k )A ∈ Y2 , Λ2k+1 = α−1 Λ2k otherwise (k = 0, 1, . . . ),

$ Λ2k =

∅ βΛ2k−1

if uµ(Λ2k−1 )A ∈ Y2 , otherwise.

(k = 1, 2, . . . ). Note that µ(Λ2k ) = α−k β k and µ(Λ2k+1 ) = α−k−1 β k (k ∈ N), so that if l+1 l  γl = µ(Λl ) (l ∈ N), one can write γl = α−[ 2 ] β [ 2 ] (as usual, [r] denotes the integer part of a real number r). Let m be the minimal integer for which Λm = ∅. Then the σ ∗ -polynomials 
A = γ v2 γk A (k = 0, . . . , m) belong to the characteristic set A, the leader uγm lies in the second quadrant, and the leaders uγl
A (0 ≤ l ≤ m − 1) lie in the first quadrant if l is even, and in the third quadrant if l is odd. Similarly, if we replace α by β and β by α, we will find a number n ∈ N and elements γ0 , . . . , γn ∈ Γ such that σ ∗ -polynomials γi A (0 ≤ i ≤ n) lie in the characteristic set A, and the leader uγn

A = γn v4 of the σ ∗ -polynomial γn A lies in the fourth quadrant. Let us now consider two cases. Case I. Suppose that at least one of the following three conditions holds: (a) max(m, n) > 1; (b) v2 = v3 ; (c)) v3 = v4 .
In this case, we have a characteristic set A = {γl A | 0 ≤ l ≤ m} {γk A | 0 ≤ ∗ ∗ k ≤ n} of the σ -ideal P . The set of all leaders of σ -polynomials in A, which we denote by UA , is as follows:      v1 | 0 ≤ 2k < m} {γ2l v1 | 0 ≤ 2l < n} {γ2p+1 v3 | 0 ≤ 2p + 1 < m} UA = {γ2k    {γ2q+1 v3 | 0 ≤ 2q + 1 < n} {γn v4 }. (7.8.1)

7.8. DIMENSION POLYNOMIALS FOR TWO TRANSLATIONS

459

Case II. Suppose that none of the conditions (a) - (c) of Case I hold. Then we have a characteristic set   A = {γl A | 0 ≤ l ≤ m} {γk A | 0 ≤ k ≤ n} {α−1 β −1 A} of the ideal P and the corresponding set UA , consisting of all leaders of σ ∗ polynomials of A, is obtained by adjoining the term α−1 β −1 v3 to the set (7.8.1). By Theorem 4.2.17, the dimension polynomial of the σ ∗ -ideal P coincides with the standard Z-dimension polynomial φB (t) of the set B = {τ (u) | u ∈ UA } ⊆ Z2 . In Case I, when elements vi (1 ≤ i ≤ 4) are pairwise distinct, the (2) set B does not contain points with zero coordinates. Setting Bi = B ∩ Zi we obtain that Bi = {τ (u) ∈ B | u = γvi for some γ ∈ Γ} (1 ≤ i ≤ 4). By Theorem 1.5.23, theleading coefficient of the dimension polynomial 4 φB (t) = a1 t + a0 is equal to j=1 (bj1 + bj2 ) − 4 where bjk = min{τ (u)k | τ (u) ∈ Bk } (k =   1, 2; 1 ≤ j ≤ 4). Note that  if max{m, n} > 1, then b11 = |τ (v1 )1 | − m−1 n−1   , b12 = |τ (v1 )2 | − , b21 = |τ (γm v2 )1 |, b22 = |τ (γm v2 )2 |, b31 = 2 2     m n |τ (v3 )1 | − + 1 , b32 = |τ (v3 )2 | − + 1 , b41 = |τ (γn v4 )1 |, and b42 = 2 2 |τ (γn v4 )2 |. If m = n = 1, then b11 = |τ (v1 )1 |, b12 = |τ (v1 )2 |, b21 = |τ (v2 )1 | + 1, b22 = |τ (v2 )2 |, b31 = |τ (v3 )1 | + 1, b32 = |τ (v3 )2 | + 1, b41 = |τ (v4 )1 |, b42 = |τ (v4 )2 | + 1. In both cases the leading coefficient of the polynomial φB (t) is of the form a1 =

4  i=1

di =

σ ∗ -ord A . 2

Now let us consider the case v1 = v2 , v3 =v4 , and n > 1. Then B contains 4 one point with a zero coordinate, hence a1 = j=1 (bj1 + bj2 ) − 3 (see Theorem σ ∗ -ord A . It is easy to 1.5.23). Therefore, in this case we also obtain that a1 = 2 see that every other case, where some of the terms v1 , . . . , v4 are equal, leads to the same expression for a1 (we leave the details to the reader as an exercise). Thus, the σ ∗ -dimension polynomial of the σ ∗ -ideal P = [A]∗ is of the form σ ∗ -ord A t + a0 , where a0 ∈ Z. It follows that σ ∗ -type P = 1 and ΨP (t) = 2∗ σ -ord A .  σ ∗ -t.dim P = 4 Corollary 7.8.7 Let K be an inversive difference field with a basic set σ = {α, β} and let P be a σ ∗ -ideal of the ring K{y}∗ , generated by one linear σ ∗ polynomial A ∈ K{y}∗ \K. Let k be the minimal element of the set of all numbers l ∈ N such that the ring K[{γy | γ ∈ Γ(l)}] contains some element of the form γ  A where γ  ∈ Γ. Then σ ∗ -t.dim P ≤ k. PROOF. Since [A]∗ = [γA]∗ for every element γ ∈ Γ, without loss of generality we may assume that A ∈ K[{γy | γ ∈ Γ(k)}]. In this case every point (p, q)

460

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

of the basic rectangle πA of the σ ∗ -polynomial A satisfies the condition |p| + |q| ≤ k, whence σ ∗ -ord A ≤ 4k. Applying Theorem 7.8.6 we obtain that σ ∗ σ ∗ -ord A ≤ k.  t.dimP = 4 Corollary 7.8.8 Let K be an inversive difference field with a basic set σ = {α, β}, K{y1 , . . . , ys }∗ the ring of σ ∗ -polynomials in σ-indeterminates y1 , . . . , ys over K and P a σ ∗ -ideal of this ring generated by one linear σ ∗ -polynomial A ∈ K{y1 , . . . , ys }∗ \ K. Let B denote the σ ∗ -polynomial obtained by replacing every term γyj (γ ∈ Γ, 2 ≤ j ≤ s) of the polynomial A with γy1 . Then the σ ∗ -dimension polynomial ΨP (t) of P has the form   ∗ 

σ -ord B t+1 t+2 +b (7.8.2) + ΨP (t) = 4(s − 1) − 4s + 4 1 2 2 where b ∈ Z. PROOF. By Corollary 2.4.12, the set A consisting of the minimal elements of the set {γA | γ ∈ Γ} with respect to the preorder  is a characteristic set of the σ ∗ -ideal [P ]∗ . (Recall that if A and B are two σ ∗ -polynomials in K{y1 , . . . , ys }∗ , then A  B if and only if the leader uB is a transform of uA .) For every j = 1, . . . , s, let Bj = {(p, q) ∈ Z2 | αp β q yj is a leader s of some ∗ σ -polynomial in A}. Then Theorem 4.2.5(iv) shows that ΨP (t) = j=1 φBj (t) where φBj (t) is the standard Z-dimension polynomial of the set B (1 ≤ j ≤ s). 4  Now, let us represent each set Bj (1 ≤ j ≤ s) in the form Bj = Bji where i=1

Bji is the family of all points of the set Bj that lie in ith quadrant (1 ≤ i ≤ 4). It is easy to see (as in the corresponding part of the proof of Theorem 7.8.6) that all leaders αp β q yj of σ ∗ -polynomials of A, for which the corresponding points (p, q) lie in the same quadrant, are determined by one of the terms of the σ ∗ -polynomial A. Therefore, if we fix i (1 ≤ i ≤ s), then Bji is not empty for ∗ only one index j (1 ≤ j ≤ s). Thus, if Q is a σ ∗ -ideal of the ring s K{y1 , . . . , ys } ∗ generated by the σ -polynomial B, then ΨP (t) = ΨQ (t) = j=1 φBj
(t) where Bj = {(p, q) ∈ Z2 | the term αp β q yj is the leader of some σ ∗ -polynomial in the characteristic set of Q described in Corollary 2.4.12} (1 ≤ j ≤ s). Since B ∈ K{y1 }∗ , Bj = ∅ for j = 2, . . . , s. Therefore (see Theorem  

t+1 t+2 + 1 (2 ≤ j ≤ s). Since φB1
(t) = −4 1.5.17(iv)), φBj
(t) = 4 1 2

 σ ∗ -ord B t + 1 + b with b ∈ Z (see Theorem 4.2.5), we obtain formula 2 1 (7.8.2).  We complete this section with several examples of computation of dimension polynomials associated with systems of linear σ ∗ -polynomials. In each of the examples we construct a characteristic set of the corresponding linear σ ∗ -ideal of the ring of σ ∗ -polynomials and then apply the formula in the last part of

7.8. DIMENSION POLYNOMIALS FOR TWO TRANSLATIONS

461

Theorem 4.2.5. The advantage of this method (in comparison with the method based on the construction of a free resolution of the corresponding module of differentials) is that it can be applied to any (not necessarily symmetric) system of linear σ ∗ -equations. Example 7.8.9 Let K be an inversive difference field of zero characteristic with a basic set σ = {α1 , α2 } and let K{y}∗ be the ring of σ ∗ -polynomials in one σ ∗ -indeterminate y over K. Let us consider the σ ∗ -equation [a1 (α1 + α1−1 − 2) + a2 (α2 + α2−1 − 2)]y = 0

(7.8.3)

where a1 and a2 are constants of the field K. Note that the dimension polynomial of such an equation was computed in Example 7.7.7 (see also Example 7.7.6) with the use of the free resolution of the σ ∗ -L-module of differentials ΩL|K where L is the σ ∗ -field of quotients of the factor ring K{y}∗ /[A]∗ , A = [a1 (α1 + α1−1 − 2) + a2 (α2 + α2−1 − 2)]y. It is easy to see that the basic rectangle πA of the σ ∗ -polynomial A is a square with the vertices (1, 0), (0, 1), (−1, 0) and (0, −1). It follows that σ ∗ -ord A = 8. Applying Theorem 7.8.6 we obtain that the σ ∗ -dimension polynomial of our σ ∗ equation (i.e., the σ ∗ -dimension polynomial of the σ ∗ -ideal [A]∗ ) has the form Ψ(t) = 4t + b where b ∈ Z. Therefore, σ ∗ -dim [A]∗ = 0, σ ∗ -type [A]∗ = 1, and σ ∗ -t.dim [A]∗ = 2. Furthermore, Corollary 2.4.12 shows that there is a characteristic set of the σ ∗ -ideal [A]∗ consisting of the σ ∗ -polynomials A, α1−1 A = [a1 (1 + α1−2 − 2α1−1 ) + a2 (α1−1 α2 +α1−1 α2−1 −2α1−1 )]y, and α1−1 α2−1 A = [a1 (α2−1 +α1−2 α2−1 −2α1−1 α2−1 )+ a2 (α1−1 + α1−1 α2−2 − 2α1−1 α2−1 )]y whose leaders are the terms α1 y, α1−1 α2 y, and α1−1 α2−2 y, respectively. Thus, the σ ∗ -dimension polynomial Ψ(t) of the σ ∗ -equation (7.8.3) coincides with the standard Z-dimension polynomial of the set B = {(1, 0), (−1, 1), (−1, −2)} ⊆ Z2 . Applying Theorem 1.5.14(iii) and formula (1.5.4) for the computation of Kolchin polynomials of subsets of Nm we obtain that Ψ(t) = φB (t) = 4t. Exercise 7.8.10 Let K be an inversive difference field of zero characteristic with a basic set σ = {α1 , α2 } and K{y}∗ the ring of σ ∗ -polynomials in one σ ∗ -indeterminate y over K. Let us consider a σ ∗ -equation [a1 (α1 − 1) − a2 (α1 α2 + α1 α2−1 + α2 + α2−1 − 2α1 − 2)]y = 0

(7.8.4)

where a1 and a2 are constants in K. (This equation is obtained by the finite difference approximation of the diffusion equation using the Krank and Nicolson scheme, see [160, Chapter 8, Section 2]). a) Find the basic rectangle of the σ ∗ -polynomial A = [a1 (α1 −1)−a2 (α1 α2 + α1 α2−1 + α2 + α2−1 − 2α1 − 2)]y and use Theorem 7.8.6 to show that the σ ∗ dimension polynomial of equation (7.8.4) is of the form Ψ(t) = 6t + b. Find σ ∗ -dim [A]∗ , σ ∗ -type [A]∗ , and σ ∗ -t.dim [A]∗ .

462

CHAPTER 7. SYSTEMS OF DIFFERENCE EQUATIONS

b) Use Corollary 2.4.12 to show that the σ ∗ -polynomials A, α1−1 A, α2−1 A, and form a characteristic set of the σ ∗ -ideal [A]∗ . Conclude that Ψ(t) = φB (t), where B = {(1, 1), (−1, 1), (1, −2), (−1, −2)} ⊆ Z2 , and use Theorem 1.5.14(iii) and formula (1.5.4) to show that Ψ(t) = 6t − 1.

α1−1 α2−1 A

Example 7.8.11 Let K be an inversive difference field of zero characteristic with a basic set σ = {α1 , α2 } and let K{y1 , y2 }∗ be the ring of σ ∗ -polynomials in one σ ∗ -indeterminates y1 and y2 over K. The finite difference approximation of the wave equation leads to the system of σ ∗ -equations of the form  a1 (α1 − 1)y1 − a2 (α2 − α2−1 )y2 = 0, a2 (α2 − α2−1 )y1 − a1 (α1 − 1)y2 = 0 where a1 and a2 are constants in K. Let P denote the σ ∗ -ideal [A1 , A2 ]∗ where A1 = a1 (α1 −1)y1 −a2 (α2 −α2−1 )y2 and A2 = a2 (α2 − α2−1 )y1 − a1 (α1 − 1)y2 . The application of Corollary 2.4.12 leads to a characteristic set of the ideal P consisting of the σ ∗ -polynomials A1 , A2 , A3 = a1 (α1 − 1)A1 − a2 (α2 − α2−1 )A2 = [a21 (α1 − 1)2 − a22 (α2 − α2−1 )2 ]y1 , A4 = α2−1 A1 = a1 (α1 α2−1 − α2−1 )y1 − a2 (1 − α2−2 )y2 , A5 = α1−1 A2 = a2 (α1−1 α2 − α1−1 α2−1 )y1 − a1 (1 − α1−1 )y2 , A6 = α1−1 α2−1 A2 = a2 (α1−1 − α1−1 α2−1 )y1 − a1 (α2−1 − α1−1 α2−1 )y2 . The leaders of these σ ∗ -polynomials are α2 y2 , α1 y2 , α12 y1 , α2−2 y2 , α1−1 α2 y1 , and α1−1 α2−2 y1 , respectively. By Theorem 4.2.5, the σ ∗ -dimension polynomial Ψ(t) of our system of algebraic difference equations can be represented as a sum of the standard Z-dimension polynomials φF1 (t) and φF2 (t) of the subsets F1 = {(2, 0), (−1, 1), (−1, −2)} and F2 = {(0, 1), (1, 0), (0, −2)} of Z2 , respectively. Applying Theorem 1.5.14 and Corollary 1.5.8 (formulas (1.5.6) and (1.5.4)) we obtain that φF1 (t) = 6t − 1 and φF2 (t) = 2t + 1. Therefore, Ψ(t) = 8t.

Chapter 8

Elements of the Difference Galois Theory In this chapter we consider some basic aspects of the difference Galois theory. The first section is devoted to the study of Galois groups of normal and separable (but not necessarily finite) difference field extensions and the application of the results this study to the problems of compatibility and monadicity. The corresponding theory was developed by P. Evanovich in [60]. The other two sections of the chapter present a review of fundamentals of two approaches to the Picard-Vessiot theory of ordinary difference field extensions. The original version of this theory, which adjusts the main ideas of the the Picard-Vessiot theory of differential fields to difference case, is due to C. Franke. In section 8.2 we give a review of this approach omitting proofs of most of the results (we prove just a few fundamental statements on Picard-Vessiot extensions). The detail exposition of the theory can be found in the fundamental C. Franke’s work [67], in his further papers [68]–[73], and also in the works by R. Infante [88] and [90]. Section 8.3 provides a review of the basics of a Galois theory of difference equations developed by M. Singer and M. van der Put. This theory, which is based on the study of Picard-Vessiot difference rings, is perfectly presented in the monograph [159] where the reader can also find interesting applications of the results on difference Galois groups to the analytic theory of difference equations.

8.1

Galois Correspondence for Difference Field Extensions

Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and L a σ ∗ -overfield of K. As before, Gal(L/K) and Galσ (L/K) will denote the Galois group and difference (or σ-) Galois group of the extension L/K, that is, the groups of all K-automorphisms and all difference (σ-) K-automorphisms of the 463

464 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY field L, respectively. We will also consider induced automorphisms α¯i (1 ≤ i ≤ n) of the group Gal(L/K) (α¯i : θ → αi−1 θαi for any θ ∈ Gal(L/K) ). Recall that a subgroup B of Gal(L/K) is said to be σ-stable if α ¯ i (B) = B (i = 1, . . . , n) and σ-invariant if α¯i (b) = b for every b ∈ B, αi ∈ σ. As we have seen, if F/K is a σ ∗ -field subextension of L/K, then Gal(L/F ) = {θ ∈ Gal(L/K) | θ(a) = a for every a ∈ F } is a σ-stable subgroup of Gal(L/K). Also, if B is a σ-stable subgroup of Gal(L/K), then {a ∈ L|θ(a) = a for every θ ∈ B} is a σ ∗ -overfield of K. This field will be denoted by L(B). In what follows, the group Gal(L/K) is considered as a topological group with the Krull topology. Recall that a fundamental system of neighborhoods of the identity in this topology is the set of all groups Gal(L/F ) ⊆ Gal(L/K) such that F is a subfield of L which is a Galois extension of K of finite degree. (When L is of finite degree over K, the Krull topology is discrete.) The topological group G = Gal(L/K) is compact, Hausdorff, and has a basis at the identity consisting of the collection of invariant, open (and hence closed and of finite index in G) subgroups of G (see Proposition 1.6.53). If H is a closed normal σ-stable subgroup of G, then each α ¯ i induces a topological automorphism βi on G/H such that βi (gH) = α ¯ i (g)H for any g ∈ G. Now one can obtain the following theorem on the Galois correspondence of difference fields in the same way as its classical analog, Theorem 1.6.54 (see the proof in [144, Section 17]). Theorem 8.1.1 Let L, K, and G = Gal(L/K) be as above. Then (i) The mapping F → Gal(L/F ) establishes a 1-1 correspondence between the set of σ ∗ -field subextensions of L/K and the set of closed σ-stable subgroups of G. If F/K is a σ ∗ -field subextension of L/K, then L(Gal(L/F )) = F , and if H is a closed σ-stable subgroup of G, then Gal(L/L(H)) = H. (ii) Let F/K be a σ ∗ -field subextension of L/K such that F is normal over K. Let γ1 , . . . , γn be the automorphisms of the group Gal(F/K) induced by α1 , . . . , αn (treated as automorphisms of F/K), respectively. Then there exists a natural isomorphism of topological groups φ : G/Gal(L/F ) → Gal(F/K) such that φβi = γi φ (1 ≤ i ≤ n) where βi denotes the automorphism of G/Gal(L/F )  induced by α ¯i . In what follows, while dealing with extensions of inversive difference fields we use the notation of section 2.1. If K is an inversive difference field with a basic set σ = {α1 , . . . , αn }, M a σ ∗ -field extension of K and Γ the free commutative group generated by σ, then for any γ ∈ Γ, the automorphism g → γ −1 gγ of the Galois group Gal(M/K) will be denoted by γ¯ . (Clearly, if γ = α1k1 . . . αnkn with ¯ 1k1 . . . α ¯ nkn .) The material presented below is based on k1 , . . . , kn ∈ Z, then γ¯ = α the results of P. Evanovich [60]. Theorem 8.1.2 Let K be an inversive difference field with a basic set σ = {α1 , . . . , αn } and let M be a σ ∗ -overfield of K such that the field extension ¯ n be autoM/K is normal and separable. Let G = Gal(M/K) and let α ¯1, . . . , α morphisms of G induced by α1 , . . . , αn , respectively. Furthermore, let Γ denote the free commutative group generated by α1 , . . . , αn . Then:

8.1. GALOIS CORRESPONDENCE

465

(i) An intermediate σ ∗ -field L of M/K is a finitely generated σ ∗ -field extension of K if and only if there exists an open subgroup H of G such that # γ¯ (H) = Gal(M/L). γ∈Γ

(ii) Let L be an intermediate σ ∗ -field of M/K such that L/K is normal and -field extension of K. Then there exists a normal L is a finitely generated σ ∗# subgroup H of G such that γ¯ (H) = Gal(M/L). γ∈Γ

PROOF. Suppose that L = KS∗ for some finite set S ⊆ L, and let A = Gal(M/K(S)). Then A is a # closed subgroup of G and since K(S) : K < ∞, we g −1 Ag. Then B is a closed normal subgroup of have G : A < ∞. Let B = g∈G

finite index in G. Since G is compact, the subgroup B is open. Denoting the group Gal(M/L) by C we obtain that # #  γ(S))) = Gal(M/K(γ(S))) = γ¯ (A) C = Gal(M/K( ⊇

#

γ∈Γ

γ∈Γ

γ∈Γ

γ¯ (BC) ⊇ C.

γ∈Γ

It follows that H = BC is and open subgroup of G and

#

γ¯ (H) = C. If

γ∈Γ

the field extension L/K is normal, then C is a σ-invariant subgroup of G and H is normal. # Conversely, suppose that H is an open subgroup of G such that Gal(M/L) = γ¯ (H). Then L(H) : K < ∞ and since Gal(M/L) ⊆ H, L(H) = K(S) for γ∈Γ

some finite set S ⊆ L. Since L is the fixed field of the group L = K(



#

γ¯ (H), we have

γ∈Γ

γ(S)), so that L = KS∗ . This completes the proof of both parts

γ∈Γ

of our theorem.



Exercise 8.1.3 With the notation of Theorem 8.1.2, suppose that n > 1. Show that if H is a σ-invariant subgroup of the Galois group G = Gal(M/K), L = kn−1 | k1 , . . . , kn−1 ∈ Z}, and σ  = {α1 , . . . , σn−1 }, then for every ∩{α1k1 . . . αn−1 k ∈ Z, the field α ¯ n (L) is σ  -stable. Exercise 8.1.4 With the notation of Theorem 8.1.2, show that M is a finitely generated σ ∗ -field extension of K if and only if G has an open invariant subgroup H such that ∩γ∈Γ γ(H) = {e} where e is the unity of the group G. Definition 8.1.5 Let M be a difference field with a basic set σ and N a σ-overfield of M . The extension N/M is said to be universally compatible if given a σ-field extension Q/M , there exist a σ-field extension R/M and σ-homomorphisms φ : N/M → R/M and ψ : Q/M → R/M .

466 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY The following three statements are immediate consequences of the definitions. We leave the proofs to the reader as an exercise. Proposition 8.1.6 If a difference (σ-) field extension L/K is universally compatible, then any its σ-field subextension F/K is universally compatible.  Proposition 8.1.7 Let K be a difference (σ-) field, L a σ-overfield of K, and M a σ-overfield of L. If the extensions M/L and L/K are universally compatible, then M/K is also universally compatible.  Proposition 8.1.8 Let K be an inversive ordinary difference field with a basic set σ = {α} and let L be a σ-overfield of K such that the field extension L/K is normal and separable. If H is an open normal subgroup of G such that α(H) ¯ ⊆ H, then H is σ-stable.  In what follows we concentrate on the questions of universal compatibility and monadicity of ordinary difference field extensions which are normal and separable. The corresponding results are due to P. Evanovich (see [60]). Proposition 8.1.9 Let K be an ordinary inversive difference field with a basic set σ = {α} and L a σ ∗ -overfield of K such that L/K is a Galois extension. Furthermore, let G = Gal(L/K), H = Galσ (L/K), and λ the mapping of the ¯ (θ) (θ ∈ G). Then group G into itself defined by λ(θ) = θ−1 α (i) λ is a continuous function. (ii) H is a closed subgroup of G. (iii) The extension L/K is universally compatible if and only if λ maps G onto itself. PROOF. It is easy to see that λ is the composition of the continuous maps µ:G→G×G

and

ν :G×G→G

defined by µ(θ) = (θ−1 , α(θ) ¯ ) and ν(θ1 , θ2 ) = θ1 θ2 , respectively. Therefore, λ is continuous. Furthermore, H = λ−1 (e), where e is the identity of the group G, hence H is a closed subgroup in G. The last statement is the direct consequence of Theorem 5.6.31.  Theorem 8.1.10 Let K be an ordinary inversive difference field with a basic set σ = {α} and L a σ ∗ -overfield of K such that L/K is a Galois extension. Then L/K is universally compatible if and only if every σ-subextension of L/K, which is a finite Galois extension of K, is universally compatible. PROOF. Let G = Gal(L/K) and let λ be the mapping of G into itself ¯ (θ)). Suppose that L/K is not unidefined in Proposition 8.1.9 (λ : θ → θ−1 α versally compatible. Then there exists an element θ ∈ G such that θ ∈ / λ(G) (see Proposition 8.1.9(iii) ). Since the group G is compact and Hausdorff, and the map λ is continuous, θ−1 λ(G) is a closed subset of G. Furthermore, the

8.1. GALOIS CORRESPONDENCE

467

identity e of the group G does not lie in θ−1λ(G), hence there exists an open normal subgroup N of G such that θ−1 λ(G) N = ∅. Let H be maximal among all open normal subgroups N of G which are disjoint with θ−1 λ(G). We are going to show that the group H is σ-stable. Taking into account Proposition 8.1.8, one just needs to show that α(H) ¯ ⊆ H. Assuming that this is not so, we obtain that H α ¯ (H) is a proper open normal  ¯ (H) = ∅, so subgroup of G properly containing H. Therefore, θ−1 λ(G) H α ¯ (h2 ), hence there exist elements γ ∈ G and h1 , h2 ∈ H such that θ−1 λ(γ) = h1 α ¯ (γ)¯ α(h−1 ) = h . Since the subgroup H is normal, there exists g∈H θ−1 γ −1 α 1 2 −1 −1 −1 −1 −1 −1 = gθ . Therefore, gθ h γ α ¯ (γh ) = gθ λ(γh such that θ−1 h−1 2 2 2 2 ) =  −1 −1 −1 ¯ ⊆ H, h1 θ λ(γh2 ) = g h1 ∈ θ λ(G) H = ∅, a contradiction. Thus, α(H) so that the subgroup H is σ-stable. Clearly, L(H)/K is a finite Galois σ-field subextension of L/K. Notice now, that there is no  γ ∈ G such that θH = λ(γ)H. (Indeed, if θH = λ(γ)H, then θ−1 λ(γ) ∈ H θ−1 λ(G). ) It follows that the finite Galois extension L(H)/K is not universally compatible. The converse statement is a direct consequence of Proposition 8.1.6.  In what follows K denotes an inversive ordinary difference field with a basic set σ = {α} and L is a σ ∗ -overfield of K such that L/K is a Galois (that is, normal and separable) field extension. Furthermore, G will denote the Galois group Gal(L/K) and e will denote the unity of G. We say that the group G is of finite type if the σ ∗ -field extension L/K is finitely generated (that is, L = KS∗ for a finite set S). Definition 8.1.11 The intersection of all open σ-stable normal subgroups of G is called the core of the group G. It is denoted by C(G). It is easy to see that C(G) is a closed σ-stable normal subgroup of G and G is the intersection of all open σ-stable subgroups of G. Exercise 8.1.12 Prove that L(C(G)) is the core of the extension L/K in the sense of Definition 4.3.17. Proposition 8.1.13 If G is of finite type and C(G) = {e}, then the group G is finite. PROOF. Let G be the set of all open σ-stable normal subgroups of G. Let ∞ # α ¯ k (N ) = {e}. Since the us choose an open subgroup N of G such that k=0

group G is compact and the elements of G are closed, there exist subgroups n # Hj ⊆ N . Clearly, H is σ-stable, hence H1 , . . . , Hn ∈ G such that H = {e} = C(G) ⊆ H ⊆

∞ # k=0

Card G = (G : H) < ∞.

j=0

α ¯ k (N ) = {e}. Since H is open in G, we obtain that 

468 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY Corollary 8.1.14 If the group G is of finite type, then (i) (G : C(G)) < ∞. (ii) C(C(G)) = C(G). PROOF. It follows from Theorem 4.4.1 that the factor group G/C(G) is of finite type. Furthermore, one can easily see that the core of G/C(G) with respect to the automorphism induced on G/C(G) by α ¯ is C(G). Applying Proposition 8.1.13 we obtain that (G : C(G)) < ∞. Statement (ii) follows from the fact that if H is an open normal subgroup of C(G), then (G : H) = (G : C(G))(C(G) : H) < ∞, hence H = C(G).  Corollary 8.1.15 Even without the requirement that G is a group of finite type, one has C(C(G)) = C(G). PROOF. By Corollary 8.1.14, if G is a group of finite type, then C(C(G)) = C(G). Let us consider the general case (G is not necessarily a group of finite type). Let C = C(G) and let C contain a proper open σ-stable normal subgroup H.  Then there exists an open normal subgroup N of G such that A = N C ⊆ H. k (A) ⊆ H for every k ∈ Z. Let A be the subgroup of H Since H is σ-stable, α ¯ generated by the set α ¯ k (A) (clearly, this is a σ-stable normal subgroup of G) k∈Z

and let B denote the closure of A in G. Since the closure of a σ-stable subgroup is obviously σ-stable, B is a closed σ-stable normal subgroup of G contained in H. Furthermore, (C : B) < ∞. It is easy to see that C(G/B) = C/B = B and one can replace G with G/B and α ¯ by the automorphism induced on G/B by α ¯ to reduce our considerations to the case where C is a nontrivial finite subgroup of G. Under this reduction, {e} is an open subgroup of G and there  is an open  α ¯ k (D) is a normal subgroup D of G such that D C = {e}. Then P = k∈Z

closed σ-stable normal subgroup of G and G/P is of finite type. We are going to show that C(G/P ) = CP/P  . Obviously, CP/P ⊆ C(G/P ). If CP = G, then G/P = CP/P ∼ = C/C P = C ⊆ {e}, hence (G : P ) < ∞ (that is, P is open in G). Since C is the intersection  of all open σ-stable subgroups of G, we obtain that C ⊆ P , hence {e} = P C = C contradicting the fact that C = {e}.  Thus, CP
G, so there is an element θ ∈ G \ CP . Then θP C = ∅. Furthermore, it follows from the definition of C and compactness of the group G that there exist a finite number of open σ-stable subgroups N1 , . . . , Nq of q q # # G such that θP Ni = ∅. Let E = Ni . Then EP/P is an open σi=1

i=1

stable subgroup of G/P not containing θP . Therefore, CP/P ⊇ C(G/P ), hence CP/P = C(G/P ).

8.1. GALOIS CORRESPONDENCE

469

 Thus, G/P is a G/P is a group of finite type with core CP/P ∼ = C/C P = C which is finite. Then P/P is an open σ-stable normal subgroup of C(G/P ) that contradicts Corollary 8.1.14.  Theorem 8.1.16 With the assumptions made before Definition 8.1.11, the extension L/K is universally compatible if and only if the σ ∗ -field extension L(C(G))/K is universally compatible. PROOF. Let C = C(G). The result of Corollary 8.1.15 implies that the extension L/L(C) has no finite σ ∗ -field subextensions. Applying Theorem 8.1.10 we obtain that the extension L/L(C) is universally compatible. Now our statement follows from Proposition 8.1.7.  It follows from the definition of a monadic extension (see Definition 5.6.1) that any Galois (and therefore normal) difference (σ-) field extension L/K is monadic if and only if Galσ (L/K) is the identity group. This observation implies the following result. Theorem 8.1.17 With the above notation and assumptions (L/K is a Galois σ ∗ -field extension of an inversive ordinary difference field with a basic set σ = {α}, G = Gal(L/K), and λ is a mapping of G into itself such that λ(θ) = ¯ (θ) = θ−1 α−1 θα for any θ ∈ G), the extension L/K is monadic if and θ−1 α only if λ is injective.  Corollary 8.1.18 With the assumptions of the last theorem, if the extension L/K is monadic, then any its σ ∗ -field subextension F/K is monadic. PROOF. The statement immediately follows from Theorem 8.1.17, since if the map λ is one-to-one on the group G = Gal(L/K), then λ is one-to-one on any subgroup of G.  Lemma 8.1.19 With the assumptions of Theorem 8.1.17, let N be an open # k α ¯ (N ) = {e}. The the natural normal subgroup of G = Gal(L/K) such that k∈Z

group homomorphism π : Galσ (L/K) → G/N is injective (and therefore, if L/K is a finitely generated σ ∗ -field extension, then Galσ (L/K) is a finite group). PROOF. If θ# ∈ Ker π, then θ ∈ N hence θ = α ¯k ∈ α ¯ k (N ) for all k ∈ Z. It k follows that θ ∈ α ¯ (N ) = {e}. Thus, π is injective.  k∈Z

Proposition 8.1.20 Keeping the previous notation and assumptions, suppose that a σ ∗ -field extension L/K is finitely generated. Then the extension L/K is monadic if and only if every its σ ∗ -field subextension F/K with L : F < ∞ is monadic. PROOF. Suppose that any σ ∗ -field subextension F/K of L/K with L : F < ∞ is monadic. If L/K is not monadic, then Galσ (L/K) = {e}. By Lemma 8.1.19, the group H = Galσ (L/K) is finite and L : L(H) < ∞. Since L/L(H) is not monadic, we obtain a contradiction with our assumption. The converse statement follows from Corollary 8.1.18. 

470 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY Proposition 8.1.21 Let L/K be as above and let F/K be a σ ∗ -field subextension of L/K such that F/K is Galois. If the extension L/K is monadic and L/F is universally compatible, then F/K is monadic. PROOF. Let G = Gal(L/K) and H = Gal(L/F ). Then the map λ : G → G defined in Proposition 8.1.9 is injective and maps H onto itself (see Theorem 8.1.17 and Proposition 8.1.9(iii) ). Let us show that the map λ : G/H → G/H, defined by λ (θH) = λ(θ)H for any θ ∈ G, is injective. Then Theorem 8.1.17 will imply that F/K is monadic. Let θH ∈ Ker λ , that is, λ(θ) ∈ H. Since λ maps H onto itself, there exists h ∈ H such that λ(θ) = λ(h). Since λ is injective on G, θ = h, so θH = H.  Thus, λ is injective, hence F/K is monadic. Theorem 8.1.22 Let K be an ordinary inversive difference field with a basic set σ = {α} and L a σ ∗ -overfield of K such that L/K is a Galois extension. Let G = Gal(L/K) and C = C(G), the core of G. If the extension L/K is monadic and L(C)/K is a finitely generated σ ∗ -field extension, then L/K is universally compatible. PROOF. As in the proof of Theorem 8.1.16 we obtain that the extension L/L(C) is universally compatible. By Proposition 8.1.21, the extension L(C)/K is monadic. Therefore, the mapping λ : H → G/H defined in the proof of Proposition 1.8.21 (with H = G/Gal(L/L(C))) is surjective, hence the extension L(C)/K is universally compatible. Applying Theorem 8.1.16 we obtain that L/K is also universally compatible.  Corollary 8.1.23 With the notation and assumptions of the last theorem, if the σ ∗ -field extension L/K is finitely generated and monadic, then L/K is universally compatible. PROOF. By Theorem 4.4.1, the extension L(C)/K is finitely generated. Now Theorem 8.1.22 implies that L/K is universally compatible.  With the above notation, if an extension L/K is not finitely generated, then the monadicity of L/K does not imply the universal compatibility of this extension. The following example of a monadic Galois difference field extension, which is not universally compatible, is due to P. Evanovich [60]. Example 8.1.24 Let us consider Q as an inversive ordinary difference field whose basic set σ consists of the identity automorphism α. Let ω1 = −1 and for n = 2, 3, . . . , let ωn denotes the primitive 2n th root of unity such that ωn2 = ωn−1 . Then ωn−1 ωn is also a primitive 2n th root of unity. For every n ∈ N, n ≥ 1, let us define inductively an automorphism αn of the field Q(ωn ) such that and αn (ωn ) = ωn−1 ωn and the restriction of αn ∞  on Q(ωn−1 ) is αn−1 (α1 = α). Let us consider the field K = Q(ωn ) as a n=1

σ ∗ -ovefield of Q whose translation (also denoted by α) is defined by the condition α = αn on Q(ωn ) (n = 1, 2, . . . ).

8.1. GALOIS CORRESPONDENCE

471

Let η1be a root of the polynomial X 2 − 5. Then it is easy to see that η∈ / K, K Q(η1 ) = Q, and the field extensions K/Q and Q(η1 )/Q are normal. Therefore (see Theorem 1.6.24), the extensions K/Q and Q(η1 )/Q are linearly disjoint. Since there exists an automorphism β of Q(η1 )/Q such that β(η1 ) = η1 , it follows from Theorem 1.6.43 that there exists an automorphism γ1 of the field K(η1 ) such that γ1 (η1 ) = η1 and γ1 extends α. Let L1 be the inversive difference field field (K(η1 ), γ1 ) treated as a σ ∗ -overfield of K. (As before, we ˆ 1 and write L1 = (L ˆ 1 , γ1 ) where γ1 will denote the underlying field of L1 by L is the translation of L1 .) The extension L1 /K is not universally compatible, since it is finitely generated and not monadic. In particular, one has a nontrivial difference automorphism φ of L1 /K defined by φ(η1 ) = −η1 . By Theorem 8.1.10, any σ ∗ -field extension of L1 is not a universally compatible extension of K. Let us inductively define a sequence of inversive difference fields {Ln | n = 1, 2, . . . } in which ˆn = L ˆ n−1 (ηn ) where every Ln is a difference field extension of Ln−1 such that L 2n 2 ηn is a zero of the polynomial X − 5, ηn = ηn−1 , and the translation γn of Ln is defined by the condition γn (ηn ) = ωnp ηn with p = 2 or p = 2n−1 + 2. ∞ ∞   ˆ= ˆ n and the translation of L, coincide with Ln (that is, L Let L = L n=1

n=1

ˆ n ). Then L/K is a Galois inversive difference field extension and by the γn on L above remark it is not universally compatible. Let us show that L/K is monadic. Let χ be a difference automorphism of L/K. Then for every n = 1, 2, . . . , the ˆ n is a difference automorphism of Ln /K. Let χ(ηn ) = ωnj ηn restriction of χ on L where j ∈ {1, 2, . . . , 2n }. Then χγn (ηn ) = χ(ωnp ηn ) = ωnp ωnj ηn where p = 2 or p = 2n−1 + 2. Furthermore, γn χ(ηn ) = γn (ωnj ηn ) = (ωn−1 ωn )j ωnp ηn . Since χγn = γn χ, one has ωnp+j = ωnp+3j , hence ωn2j = 1. It follows that ˆ n−1 is the χ(ηn−1 ) = χ(ηn2 ) = (ωnj ηn )2 = ηn−1 , so the restriction of χ on L identity automorphism. Since n is arbitrary, χ is the identity automorphism of ˆ Thus, the extension L/K is monadic. L. Theorem 8.1.25 Let K be an ordinary inversive difference field with a basic set σ = {α} and L a σ ∗ -overfield of K such that L/K is a Galois extension. Let F/K be a Galois σ ∗ -field subextension of L/K. Then (i) Galσ (L/F ) is a closed σ-stable normal subgroup of Galσ (L/K). (ii) There exists a natural monomorphism ψ : Galσ (L/K) / Galσ (L/F ) → Galσ (F/K). If L/F is universally compatible then ψ is an isomorphism. (iii) If L/K is universally compatible and ψ is an isomorphism, then L/F is universally compatible. PROOF. Let G = Gal(L/K), H = Gal(L/F ), D1 = Galσ (L/K), D2 = Galσ (L/F ), and D3 = Galσ (F/K). Statement (i) follows from the fact that H is a closed normal subgroup of G and D2 = D1 H.

472 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY To prove (ii) let us identify D3 with the subgroup of G/H consisting of the cosets θH for which λ(g) = e (λ is the mapping of the group G into itself defined in Proposition 8.1.9, e is the unity of G). If π : G → G/H is the natural homomorphism and θ ∈ D1 , then λ(θ) = e ∈ H. It follows that therestriction of π on D1 is a homomorphism of D1 into D3 whose kernel is D1 H = D2 . Let ψ denote the monomorphism of D1 /D2 into D3 induced by π. Then ψ is a continuous and closed map. If the extension L/F is universally compatible, so that λ(H) = H and θH ∈ D3 (θ ∈ G), λ(θ) ∈ H and there exists h ∈ H such that λ(h) = λ(θ). It follows that λ(θh−1 ) = e, θh−1 ∈ D1 , and ψ(θh−1 D2 ) = θH. Thus, ψ is an isomorphism of topological groups, so (ii) is proved. Suppose that ψ is an isomorphism and λ(G) = G (that is, L/K is universally compatible). Let h ∈ H. Then there exists θ ∈ G such that λ(θ) = h, gH ∈ D3 . Since ψ is an isomorphism, there exists θ1 ∈ D1 such that θ1 = θh1 for some ele¯ (θ1 ) = α(θh ¯ ¯ ((h1 ), hence λ(h−1 ment h1 ∈ H. Then θh1 = θ1 = α 1 ) = θhα 1 ) = h. Thus, λ(H) = H, so the extension L/F is universally compatible.  The following result was obtained in [60] with the use of the theory of limit groups of ordinary difference fields developed by P.Evanovich. We refer to the work [60] for the proof. Theorem 8.1.26 Let K be an ordinary inversive difference field with a basic set σ = {α} and let L/K be a finitely generated monadic Galois σ ∗ -field extension of K. Then the group Gal(L/K) is finite.

8.2

Picard-Vessiot Theory of Linear Homogeneous Difference Equations

Most of the results of this section are due to C. Franke and contained in his works [67] - [70]. In what follows we assume that all fields have characteristic zero and all difference fields are inversive and ordinary. If K is such a difference field with a basic set σ = {α}, then its field of constants {c ∈ K|α(c) = c} will be denoted by CK . All topological statements of this section will refer to the Zariski topology. As before, K{y} will denote the ring of σ-polynomials in one σ-variable y over K. Furthermore, for any n-tuple b = (b1 , . . . , bn ), C ∗ (b) or C ∗ (b1 , . . . , bn ) will denote the determinant of the matrix (αi bj )0≤i≤n−1,1≤j≤n . This determinant is called the Casorati determinant of b1 , . . . , bn . Lemma 8.2.1 With the above notation, the following two conditions are equivalent: (i) Elements b1 , . . . , bn are linearly dependent over the constant field of any difference field containing them. (ii) C ∗ (b) = 0.

8.2. PICARD-VESSIOT THEORY PROOF. If

n 

473

ci bi = 0 for some constants c1 , . . . , cn , the rows of C ∗ (b) are

i=1

linearly dependent, hence C ∗ (b) = 0. Conversely, suppose that C ∗ (b) = 0. We are going to show property (i) by induction on n. Since the case n = 1 is trivial, it is sufficient to show that if b1 , . . . , bn are elements of a difference field K such that C ∗ (b) = 0 and no proper subset of {b1 , . . . , bn } has Casorati determinant zero, then b1 , . . . , bn are linearly dependent over the constant field of any difference field containing them. Let Ai denote the cofactor of αn−1 bi in C ∗ (b) (i = 1, . . . , n). By our assumption, all Ai are different from zero, so the rank of the matrix of C ∗ (b) is n − 1. Furthermore, it is easy to see that (−1)n−1 αAi is a cofactor of bi (αAi denotes the determinant obtained by applying α to every element of the determinant Ai ). Since C ∗ (b) = 0, we obtain that A1 (αj b1 ) + · · · + An (αj bn ) = 0, αA1 (αj b1 ) + · · · + αAn (αj bn ) = 0

(j = 0, . . . , n − 1).

(8.2.1)

Since the rank of the matrix of C ∗ (b) is n − 1, these equations imply that αAi /αA1 = Ai /A1 for i = 2, . . . , n. Setting ci = Ai /A1 (i = 2, . . . , n) and n  ci bi = 0. c1 = 1 we obtain constants c1 , . . . , cn ∈ Kb1 , . . . , bn  such that i=1

This completes the proof.



Let us consider a linear homogeneous difference equation of order n over K, that is, an algebraic difference equation of the form αn y + an−1 αn−1 y + · · · + a0 y = 0

(8.2.2)

where a0 , . . . , an−1 ∈ K (n > 0) and a0 = 0. Definition 8.2.2 A σ ∗ -overfield M of K is said to be a solution field over K for equation (8.2.2) or a solution field over K for the σ-polynomial f (y) = αn y + an−1 αn−1 y + · · · + a0 y, if M = Kb∗ for an n-tuple b = (b1 , . . . , bn ) such that f (bj ) = 0 for j = 1, . . . , n and C ∗ (b) = 0. Any such n-tuple b is said to be a basis of M/K or a fundamental system of solutions of (8.2.2). If, in addition, CK is algebraically closed and CM = CK , then M is said to be a Picard-Vessiot extension (PVE) of K. (By Lemma 8.2.1, the elements b1 , . . . , bn are linearly independent over the constant field of any difference field containing them.) Proposition 8.2.3 With the above notation, let M be a σ ∗ -overfield of K and R ⊆ CM . Then (i) A subset of R linearly (algebraically) dependent over K is linearly (respectively, algebraically) dependent over CK . (ii) If N is a σ ∗ -overfield of K with CN = CK , then N and K(R) are linearly disjoint over K. (iii) CK(R) = CK (R).

474 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY PROOF. (i) Suppose that the first statement of our proposition is false and W ⊆ R is a minimal linearly dependent set over K which is linearly independent m  ki wi for over CK . Then there exist w0 , w1 , . . . , wm ∈ W such that w0 = some k1 , . . . , km ∈ K. Since 0 = α(w0 ) − w0 =

m 

i=1

(α(ki ) − ki )wi , we arrive at a

i=1

contradiction with the minimality of W . Let a set U ⊆ R be algebraically dependent over K and let V be a basis of K treated as a vector CK -space. If f (u1 , . . . , up ) = 0 for some u1 , . . . , up ∈ U and some polynomial in p variables f ∈ K[X1 , . . . , Xp ], then every coefficient of f can be written as a linear combination of elements o V . Therefore, there are some polynomials h1 , . . . , hr ∈ CK [X1 , . . . , Xp ] and elements v1 , . . . , vr ∈ V such that r  f = hj vj . Since the elements v1 , . . . , vr are linearly independent over CK , j=1

their Casorati determinant is not zero, so v1 , . . . , vr are linearly independent over CK u1 ,...,up  (see Lemma 8.2.1). Therefore, hj (u1 , . . . , up ) = 0 for j = 1, . . . , r, so u1 , . . . , up are algebraically dependent over CK . (ii) Clearly, the set of all power products of elements of R contains a basis B of the vector K-space K[R]. Since elements of B are constants, statement (i) implies that the set B is linearly independent over N . By Theorem 1.6.24, the fields N and K(R) are linearly disjoint over K. (iii) It is easy to see that if the last statement of the proposition is false, it is false for a finite set R. By induction it is sufficient to consider the case where R consists of a single element u. If u is algebraic over K, then every element d  ai ui for some w ∈ CK(R) ⊆ K(R) = K(u) can be written uniquely as w = i=0

a0 , a1 , . . . , ad ∈ K (we assume d ≥ 1, since the case R ⊆ K is trivial). Then d  0 = α(w) − w = (α(ai ) − ai )ui hence ai ∈ CK for i = 0, . . . , d. Therefore, CK(R) ⊆ CK (R).

i=0

If u is transcendental over K, the arbitrary element v ∈ CK(R) ⊆ K(u) can be f (u) . written uniquely as a ratio of two relatively prime polynomials in u: v = g(u) f α (u) f (u) − (as usual, f α denotes a polynomial obtained Then 0 = α(v) − v = α g (u) g(u) by applying α to every coefficient of the polynomial f ), hence f α (u)g(u) = f (u)g α (u). Since the polynomials f (u) and g(u) (as well as f α (u) and g α (u)) are relatively prime, f α (u) = f (u) and g α (u) = g(u), so that all coefficients of f and g are constants. Thus, in this case we also have the inclusion CK(R) ⊆ CK (R). Since the opposite inclusion is obvious, this completes the proof. 

8.2. PICARD-VESSIOT THEORY

475

Proposition 8.2.4 Let K be a difference field with a basic set σ = {α}, K{y} the ring of σ-polynomials in one σ-variable y over K, and f = αn y + an−1 αn−1 y + · · · + a0 y a homogeneous linear difference polynomial in K{y} (a0 , . . . , an−1 ∈ K, a0 = 0). Then equation (8.2.2) (whose left-hand side is f ) has n distinct solutions u1 , . . . , un such that (i) The elements u1 , . . . , un are linearly independent over the field of constant of Ku1 , . . . , un . (ii) If v is any solution of equation (8.2.2) lying in some difference overfield of Ku1 , . . . , un , then v can be written as v = c1 u1 + · · · + cn un where c1 , . . . , cn are constants of the field Ku1 , . . . , un , v. (iii) The σ-field extension Ku1 , . . . , un /K is compatible with every σ-field extension of K. PROOF. Clearly, the variety M(f ) is irreducible and ord M = n. Let u1 be a generic zero of this variety. Then we define ui (i = 2, . . . n) inductively as a generic zero of the variety of f over Ku1 , . . . , ui−1 . It is easy to see that the set {αj ui | 1 ≤ i ≤ n, 0 ≤ j ≤ n − 1} is algebraically independent over K, so the Casorati determinant of the elements of this set is not zero. Applying Lemma 8.2.1 we obtain the first statement of our proposition. If v is any solution of (8.2.2) in a σ-overfield of Ku1 , . . . , un , then αn ui + an−1 αn−1 ui + · · · + a0 ui = 0 n

α v + an−1 α

n−1

v + · · · + a0 v = 0.

(i = 1, . . . , n), (8.2.3)

It follows that the Casorati determinant of u1 , . . . , un , v is zero, so that these elements are linearly dependent over the field of constants of Ku1 , . . . , un , v. Since u1 , . . . , un are linearly independent over CK u1 ,...,un ,v , this completes the proof of statement (ii). By Proposition 5.1.12, for every i, Ku1 , . . . , ui  is a purely transcendental field extension of Ku1 , . . . , ui−1  and, therefore, of K. Applying Theorem 5.1.6 we obtain that Ku1 , . . . , un /K is compatible with every σ-field extension of the field K.  The last proposition implies that if M is a solution field for equation (8.2.2) over K with basis b = (b1 , . . . , bn ) and b is any solution of (8.2.2) in a σ ∗ n  cj bj for some elements c1 , . . . , cn ∈ CN . It overfield N of M , then b = i=1

follows that a σ-homomorphism h of K{b}/K into a σ ∗ -overfield N of M den  cij bj . termines an n × n-matrix (cij ) over CN by the equations h(bi ) = i=1

The following theorem and corollary show that the matrices corresponding to σ-homomorphisms satisfy a set of algebraic equations over CM , and, in the case of a PVE, form an algebraic matrix group.

476 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY Theorem 8.2.5 If M/K is a solution field with basis b = (b1 , . . . , bn ), then there is a set Sb in the polynomial ring CM [{Xij | 1 ≤ i, j ≤ n}] such that if N is a σ ∗ -overfield of M then the following hold. (i) A σ-homomorphism of K{b}/K to N/K (that is, a difference Khomomorphism of K{b} to N ) determines a matrix in CN that annuls every polynomial of Sb . (In the last case we say “the matrix satisfies Sb ”). (ii) A matrix in CN satisfying Sb defines a σ-homomorphism of K{b}/K to N/K. (iii) If CM = CK then a σ-homomorphism of K{b}/K to N/K determines a σ-isomorphism if and only if its matrix is nonsingular. PROOF. Let K{y1 , . . . , yn } be the ring of σ-polynomials in σ-indeterminates y1 , . . . , yn over K and let P be a prime σ ∗ -ideal of K{y1 , . . . , yn } with generic zero b. Let φ be a ring homomorphism from K{y1 , . . . , yn } to M [{Xij | 1 ≤ i, j ≤ n  n}] defined by φ(yi ) = bj Xij and let J = φ(P ). Then every polynomial h ∈ J j=1

can be written as h=



gk vk

(8.2.4)

where gk ∈ CM [{Xij | 1 ≤ i, j ≤ n}] and vk are elements of a basis V of M over CM (when M is treated as a vector CM -space). We define Sb to be the set {g ∈ CM [{Xij | 1 ≤ i, j ≤ n}] | g appears as some gk in representation (8.2.4) of some polynomial h ∈ J}. (i) If θ is a difference K-homomorphism of K{b} to N with matrix (cij )≤i,j≤n , n n   then the mappings defined by yi → bi → θ(bi ) and yi → bj Xij → bj cij j=1

j=1

are identical. Therefore, the latter mapping sends P to zero, and every polynomial in J vanishes for Xij = cij . Since the linear independence of V over CM carries over to CN , all the polynomials of Sb vanish at cij . (ii) If (cij )1≤i,j≤n is a matrix over CN satisfying Sb , then the mapping den n   fined by yi → bj Xij → bj cij is a difference K-homomorphism from j=1

j=1

K{y} to N whose kernel contains P . This homomorphism induces a difference K-homomorphism of K{b} to N . (iii) To prove the last statement of the theorem it is sufficient to show that if φ is the σ-homomorphism of K{b}/K to N/K defined by a nonsingular matrix (cij )1≤i,j≤n over CN (as it is shown in the proof of (ii)), then φ is injective. Suppose that this is not so. Then Proposition 1.6.32(v) implies that trdekK Kb1 , . . . , bn  > trdegK Kφ(b1 ), . . . , φ(bn ). (The transcendence degrees are finite, since each bi satisfies (8.2.2).) Denoting the sets {b1 , . . . , bn }, {φ(b1 ), . . . , φ(bn )} and {cij | 1 ≤ i, j ≤ n} by b, φ(b) and c, respectively, and using the additive property of transcendence degree we obtain that trdegK b Kb, φ(b) < trdekK φ(b) Kb, φ(b).

8.2. PICARD-VESSIOT THEORY

477

Furthermore, denoting the field CK = CM by C we obtain that trdegK b Kb, φ(b) = trdegK b Kb, c = trdegC C(c) . (The last equality follows from Proposition 8.2.3(i).) Similarly, trdegK φ(b) Kb, φ(b) = trdegC
C  (c) where C  denotes the field of constants of Kφ(b). We have arrived at a contradiction with the obvious inequality trdegC
C  (c) ≤ trdegC C(c). This completes the proof of the theorem.  Part (iii) of the last theorem immediately implies the following statement. Corollary 8.2.6 If M/K is a PVE then the difference Galois group Galσ (M/  K) is an algebraic matrix group over CK . If M is a solution field for equation (8.2.2) and b = (b1 , . . . , bn ) a basis of M/K, then Sb will denote the set of polynomials in Theorem 8.2.5 (Sb ⊆ CM [{Xij | 1 ≤ i, j ≤ n}] ), and Tb will denote the variety of Sb over the algebraic closure of CM . The following example shows that a matrix in Tb may not correspond to a difference homomorphism of K{b}. Example 8.2.7 ([Franke, [67]). Let b be a solution of the difference equation αy + y = 0 which is transcendental over K. Then the constant field of K(b) contains CK (b2 ), Sb = {0}, and Tb contains the algebraic closure of CK (b2 ). Since no σ ∗ -overfield of Kb∗ contains b in its constant field, Theorem 8.2.5 does not apply to the matrix (b). The algebraic isomorphism h : K{b} → K{b} defined by h(b) = b2 , is not a σ-homomorphism. Let L be an intermediate σ ∗ -field of a difference (σ ∗ -) field extension M/K (M is a solution field for equation (8.2.2) over K). A σ ∗ -overfield N of M is said to be a universal extension of M for L if every σ-isomorphism of L over K can be extended to a σ-isomorphism of M into N . It follows from Theorem 5.1.10 that if L is algebraically closed in M , then universal extensions of M for L exist. At the same time, one can show (see [70, Example 4.2]) that even if L itself is a solution field over K, M need not be a universal σ ∗ -field extension for L. Theorem 8.2.8 Let M be a solution field for equation (8.2.2) over an ordinary difference field K with a basic set σ = {α} and let b be a basis of M/K. If the field K is algebraically closed in M , then the variety Tb is irreducible and dim Tb = trdegK M . PROOF. Let us define the prime σ ∗ -ideal P of the ring of σ-polynomials K{y1 , . . . , yn } (this ring will be also denoted by K{y}) and the ring homomorphism φ from K{y} to the polynomial ring M [{Xij | 1 ≤ i, j ≤ n}] (also denoted by M [X]) as it is done in the proof of Theorem 8.2.5. Furthermore, let f denote the linear difference polynomial in the left-hand side of (8.2.2).

478 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY Let J = φ(P ) and let Sb denote the ideal generated by Sb in the polynomial ring CM [{Xij | 1 ≤ i, j ≤ n}] (this ring will be denoted by CM [X]). If P  is the perfect ideal generated by P in M {y} (that is, in the ring of σ-polynomials M {y1 , . . . , yn }), then P  is a prime σ ∗ -ideal consisting of linear combinations of elements of P with coefficients in M (see Corollary 7.1.19). Let J  = φ(P  ) and g ∈ CM [X] which appear when each h ∈ J  is let Q be the set of all polynomials  written as a finite sum h = gk vk for a vector space basis {vk } of M over CM . k

Note that φ maps M {y} onto M [X], since the equations φ(αk yi ) =



Xij αk bj

j

can be solved for the Xij showing that each Xij is in the range of φ. Let b be a generic zero of P  . Then f (b ) = 0, hence  there are constants   cij in a difference overfield N of M such that bi = cij bj . If h ∈ M [X] j  Xij bj ), hence h(cij ) = u(bi ). It and h = φ(u), then h(Xij ) = φ(h(yi )) = u( j

follows that h(cij ) = 0 if and only if u ∈ P  and J  is a prime ideal in M [X] with generic zero (cij ). Since {vk } is linearly independent over the field of constants CN , Q is a prime ideal in CM [X] with generic zero (cij ).  pj mj for some pj ∈ P and mj ∈ M . Therefore, If p ∈ P  , then p = j

any element in J  can be written as φ(p) =



φ(pj )mj where φ(pj ) ∈ J. If

j

h ∈ Q , then there exists z ∈ J  such that z = hv1 + z=



z j mj =

 j

z =





hj vj . Furthermore,

j

gij vi mj for some gij ∈ Sb . The last inequality implies that

i,j

gij dijk vijk for some dijk ∈ CM . Since every element has a unique

i,j,k

expression as a linear combination of elements of a vector space basis, h =  dijk gij where the sum is taken over all i, j, k with vijk = v1 . Therefore, h ∈ Sb and since Sb ⊆ Q , we obtain that Sb = Q . Since Q is a prime ideal, the variety Tb is irreducible. Since M/K is compatible with the extension formed by a generic zero of P , we have ord P = ord P  , hence trdegK M = ord P = ord P  = trdegM M b  = trdegM M (cij ) = trdegCM CM (cij ) = dim Q = dim Tb . This completes the proof.  Let M be a σ ∗ -overfield of a difference field K with a basic set σ. We say that the extension M/K is σ-normal if for every x ∈ M \ K, there exists a σ-automorphism φ of M such that φ(x) = x and φ(a) = a for every a ∈ K. (Note that C. Franke [67] called such extensions “normal” while similar differential field extensions are called “weekly normal”.) The existence of proper monadic algebraic difference extensions suggests the existence of solution field that are not σ-normal extensions.

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In what follows we present some further results of the Franke’s version of the Picard-Vessiot theory of difference equations. We refer the reader to the works [67] - [73] for the proofs. The next statement is a version of the fundamental Galois theorem for PVE. As usual, primes indicate the Galois correspondence: if H is a subgroup of Gal(M/K), then H  is the fixed field of H, and if L is an intermediate field of M/K, then L is a subgroup of Gal(M/K) whose elements fix all elements of the field L. Theorem 8.2.9 (Franke, [67]). Let M/K be a difference (σ ∗ -) PVE, G = Galσ (M/K), L an intermediate σ ∗ -field of M/K, and H an algebraic subgroup of G. Then (i) L is an algebraic matrix group. (ii) H  = H. (iii) If L is algebraically closed in M , then M is σ-normal over L and L = L. (iv) There is a one-to-one correspondence between intermediate σ ∗ -fields of M/K that are algebraically closed in M and connected algebraic subgroups of the group G. ¯ denote the algebraic closure of K in M . If H is a connected normal (v) Let K subgroup of G, then G/H is the full group of H  over K (that is, G/H is ¯ isomorphic to Galσ (H  /K)) and H  is σ-normal over K. (vi) If L is algebraically closed in M and σ-normal over K, then L is a normal subgroup of G and G/L is the full group of L over K. Let M be a σ ∗ -overfield of a difference (σ ∗ -) field K and H a subgroup of Galσ (M/K). If L is an intermediate σ ∗ -field of M/K, then LH will denote the group {h ∈ H|h(a) = a for all a ∈ L} ⊆ H. A subgroup A of H is said to be Galois closed in H if (A )H = A. An intermediate field N of M/K is said to   ) = N. be Galois closed with respect to H if (NH The results of the following theorem were obtained in [67] and [69]. Theorem 8.2.10 Let M be a solution field for difference equation (8.2.2) over a difference (σ-) field K. Let b be a basis of M/K and H a subgroup of Galσ (M/K) which is naturally isomorphic to the set of matrices Tb corresponding to H. Then: (i) Algebraic subgroups of H are Galois closed in H. (ii) Connected subgroups of H correspond to intermediate σ ∗ -fields of M/K algebraically closed in M . (iii) Let L be an intermediate σ ∗ -field of M/K which is algebraically closed in M . Let TbL be the variety obtained by considering M as a solution field over L with basis b. If LH is dense in TbL , then (LH ) = L and LH is connected. (iv) If the algebraic closure of K(CM ) in M coincides with K, then M/K is a σ-normal extension. In this case, there is a one-to-one correspondence between connected algebraic subgroups of Galσ (M/K) and intermediate σ ∗ -fields of M/K algebraically closed in M .

480 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY Some generalization of the last theorem was obtained in [70]. Let K and M be as in Theorem 8.2.10, L an intermediate σ ∗ -field of M/K, and N a σ ∗ -overfield of M . Furthermore, let IL denote the set of all σ-isomorphisms of M into N leaving L fixed. Proposition 8.2.11 If L is algebraically closed in M , then IL is a connected algebraic matrix group. Furthermore, Galσ (M/L) is dense in IL and IL is isomorphic to Galσ (M (CN )/L(CN )). Theorem 8.2.12 Let K and M be as in Theorem 8.2.10, H an algebraic subgroup of Galσ (M/K), and L an intermediate σ ∗ -field of M/K. Then (i) H  = H. (ii) If the field L is algebraically closed in M , then L = L and M is σ-normal over L. (iii) There is a one-to-one correspondence between connected algebraic subgroups of Galσ (M/K) and intermediate σ ∗ -fields of M/K algebraically closed in M . (iv) Assume that H is connected and L = H  . In this case (a) H is a normal subgroup of Galσ (M/K) if and only if L is σ-normal over the field K. (b) If H is a normal subgroup of Galσ (M/K) and N is any universal extension of M for L, then IL is a normal subgroup of IK . The homomorphisms defined by restriction and extension determine natural isomorphisms Galσ (M/K)/H → Galσ (L/K) → IK /IL and the image of Galσ (M/K)/H is dense in IK /IL . In general a full difference Galois group G = Galσ (M/K) is not naturally isomorphic to a matrix group (if g, h ∈ G, then the matrix of the composite of g and h is the matrix of g times the matrix obtained by applying g to the entries of the matrix of h). However, if we adjoin CM to K and consider M as a solution field over K(CM ), we obtain a group D which is naturally isomorphic to a group of matrices contained in an algebraic variety T . Theorem 8.2.5 implies that T consists only of isomorphisms and singular matrices. The Galois correspondence given in Theorem 8.2.10 for D and fields between K(CM ) and M depends in part on whether a subgroup of D is dense in a variety containing it. Examples where this is not the case are not known. If M is a solution field for (8.2.2) over K with a basis b, then the subsets of Tb and D consisting of nonsingular matrices with entries in CK are automorphism groups. The following two results obtained in [67] deal with these groups. Proposition 8.2.13 Let M be a solution field for difference equation (8.2.2) over a difference (σ-) field K. Let b be a basis of M/K, Λ a subfield of CM , and Sb the subset of the polynomial ring CM [xij ] (1 ≤ i, j ≤ n) whose existence  is established by Theorem 8.2.5. Let us write the polynomials of Sb as f = fk vk k

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where {vk } is a basis of CM as vector space over Λ and fk ∈ Λ[xij ]. Let Sb be the set of all such fk . Then (i) Every solution of the set Sb is a solution of Sb . (ii) Every solution of Sb that lies in Λ is a solution of Sb . (iii) If Λ is algebraically closed and contained in K, then the variety of Sb over Λ is an algebraic matrix group of automorphisms of M/K plus singular matrices. (1)

Note that Theorem 8.2.10 can be applied to any group Gb obtained by (1) deleting the singular matrices from a variety Tb determined as in Proposition 8.2.13 by a basis b and a subfield Λ. Proposition 8.2.14 Let K, M and b be as in Proposition 8.2.13, and let Λ be (1) an algebraically closed field of constants of K. Let Gb be the group determined (1) by b and Λ as in Proposition 8.2.13 and let Cb be the component of the identity (1) ¯ (the of Gb . Finally, let Cb be the irreducible subvariety of Tb determined by K ¯ is algebraic closure of K in M ). The following are equivalent and imply that K (1) Galois closed with respect to Cb . (1)

(i) Cb

is dense in Cb . (1)

(ii) dim Cb = dim Cb . (iii) There is a basis for the ideal of Cb in the polynomial ring C[xij ] (1 ≤ i, j ≤ n). Let M be a solution field for difference equation (8.2.2) over a difference (σ-) field K. M/K is called a generalized Picard-Vesiot extension (GPVE) if there (1) is a basis b of M/K and an algebraically closed subfield Λ of CK such that Cb is dense in Cb . M is said to be a generic solution field for equation (8.2.2) if trdegK M = n2 (n is the order of the difference equation). Proposition 8.2.15 Every linear homogeneous difference equation over a difference field K has a generic solution field M . Therefore, if CK contains an algebraically closed subfield, then every linear homogeneous difference equation over K has a solution field which is a GPVE. Theorem 8.2.16 If L = Ka∗ and M = Kb∗ are solution fields of equation (8.2.2) over a difference (σ-) field K, then trdegK(CL ) L = trdegK(CM ) M . Furthermore, if L/K and M/K are compatible, then (i) There is a difference (σ-) field M1 isomorphic to M and a set of constants R such that L(R) = M1 (R). (ii) If L is a PVE, then there is a specialization b → b with L = Kb ∗ . (iii) If L and M are PVE of K, then L and M are σ-isomorphic over K. As in the corresponding theory for differential case, three types of extensions are used in constructing solution fields for linear homogeneous difference equations over a difference field K: solution fields for difference equations αy = Ay

482 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY or αy − y = B (A, B ∈ K), and algebraic extensions. The following is a brief account of this approach (the proofs can be found in [67]). Proposition 8.2.17 Let K be a difference field with a basic set σ = {α}, 0 = B ∈ K, and let a be a solution of the difference equation αy − y = B.

(8.2.5)

Then M = K(a) is a corresponding solution field over K with basis b = (a, 1). If there is no solution of equation (8.2.5) in K, then a is transcendental over K, K(a) has no new constants, and there are no intermediate σ ∗ -fields different from K and K(a). If there is a solution f ∈ K, then K(a) is an extension of K generated by a constant which may be either transcendental

or algebraic over K. If a is 1 c where c lies in the algebraic transcendental, then Tb is the set of matrices 0 1 closure of CM . If CM = CK , then the full Galois group of M/K is isomorphic to the additive group of CK . Proposition 8.2.18 Let K be as before and let a be a nonzero solution of the difference equation αy − Ay = 0 (8.2.6) (A ∈ K). Let us consider the equation αy − An y = 0

(8.2.7)

where n ∈ N, n > 0. Then (i) If there is no nonzero solution in K of the equation (8.2.7), then a is transcendental over K and K(a) has no new constants. (ii) If L is an intermediate σ ∗ -field, then L = K(an ) for some n ∈ N. (iii) If equation (8.2.7) has a solution in K for some n ∈ N, n > 0, then K(a) is obtained from K by an extension by a constant, which may be either transcendental or algebraic over K, followed by an algebraic extension. If a is transcendental over K, the variety Ta is the full set of all constants. If CK(a) = CK , then the full Galois group of K(a)/K is a multiplicative subgroup of CK . Definition 8.2.19 Let K be a difference field with a basic set σ and N a σ ∗ overfield of K. N/K is said to be a Liouvillian extension (LE) if there exists a chain K = K0 ⊆ K1 ⊆ · · · ⊆ Kt = N, Kj+1 = Kj aj ∗ (j = 0, . . . , t − 1)

(8.2.8)

where aj is one of the following. (a) A solution of an equation (8.2.5) where B ∈ Kj and there is no solution of (8.2.5) in the field Kj . (b) A solution of an equation (8.2.6) where A ∈ Kj and for any n ∈ N, n > 0, there is no nonzero solution of (8.2.7) in the field Kj . (c) An algebraic element over Kj .

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More generally, N/K is said to be a generalized Liouvillian extension (GLE) if there exists a chain (8.2.8) where aj is either a solution of equation (8.2.5) with B ∈ Kj or a solution of equation (8.2.6) with A ∈ Kj , or an algebraic element over Kj . It follows from the definition that if a difference field K has an algebraically closed field of constants and a solution field M for a difference equation (8.2.2) is contained in a Liouvillian extension N of K, then M is a PVE of K. The following results connect the solvability of a difference equation with the solvability of a matrix group. Theorem 8.2.20 Let M be a solution field for difference equation (8.2.2) over a difference (σ-) field K. Let H be a connected group of automorphisms of M/K with matrix entries with respect to some basis b in an algebraically closed subfield of CM . (It need not be isomorphic to the set of matrices corresponding to H.) (i) If H is solvable, then M/H  is a GLE. (ii) If H is reducible to diagonal form, then M/H  can be obtained by solving equations of the type (8.2.6). (iii) If H is reducible to special triangular form, then M/H  can be obtained by solving equations of the type (8.2.5). Theorem 8.2.21 Let K be a difference field with a basic set σ = {α} and M a σ ∗ -overfield of K. (i) If M/K is a PVE, then M/K is a GLE if and only if the component of identity of the Galois group is solvable. (ii) If M is a solution field of a difference equation (8.2.2) contained in a GLE N/K, then the component of identity of Gal(M/K(CM )) is solvable. Furthermore, if M/K(CM ) is a GPVE, then M/K is a GLE. (iii) Suppose that M and L are solution fields of a difference equation (8.2.2) over K. If L is contained in a GLE N of K and M/K is compaitible with N/K, then M is contained in a GLE of K. (iv) If N is a generic solution field for (8.2.2) over K and a solution field L for (8.2.2) is contained in a GLE of K, then N is contained in a GLE of K. The following example indicates that it is not satisfactory to consider equation (8.2.2) to be “solvable by elementary operations” only if its solution field is contained in a GLE. Example 8.2.22 With the notation of Theorem 8.2.21, suppose that K contains an element j with α(j) = j and α2 (j) = j, and an element u with the α(v) for some v ∈ K, k ∈ N, then following property. If uk = vα(v) or uk = v k = 0. If η is any nonzero solution of the difference equation α2 y−uy = 0, then M = Kη∗ is a solution field for this equation with basis (η, α(η)), trdegK M = 2,

484 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY CM = CK and Galσ (M/K) is commutative. However, as it is shown in [68, Example 1], M is not a GLE of K. In what follows we consider some results by C. Franke (see [68] and [73]) that characterize the solvability of a difference equation of the form (8.2.2) “by elementary operations”. Throughout the rest of the section K denotes an inversive difference field with a basic set σ = {α}. If L is a σ ∗ -overfield of K, then KL will denote the algebraic closure of K(CL ) in L. Definition 8.2.23 Let N be a σ ∗ -overfield of K, and q a positive integer. A q-chain from K to N is a sequence of σ ∗ -fields K = K1 ⊆ K1 ⊆ · · · ⊆ Kt = N , Ki+1 = Ki ηi ∗ where ηi is one of the following. (a) An element algebraic over Ki−1 . (b) A solution of an equation αq y = y + B for some B ∈ Ki . (c) A solution of an equation αq y = Ay for some A ∈ Ki . If there is a q-chain from K to N , then N is called a qLE of K. Let K (q) denote the field K treated as an inversive difference field with a basic set σq = {αq } and let N (q) be a σ ∗ -overfield N of K treated as a σq∗ -overfield of K (q) . In this case N is a qLE of K if and only if N (q) is a GLE of K (q) (see [68, Proposition 2.1]). Theorem 8.2.24 If M is a σ-normal σ ∗ -overfield of K such that K = KM and the group Galσ (M/K) is solvable, then M is contained in a qLE of K. If, in addition, M/K is a σ ∗ -field extension generated by a fundamental system of solutions of a difference equation (8.2.2), then M is contained in a GLE of K. Theorem 8.2.25 Let N be a qLE of K and L an intermediate σ ∗ -field of N/K. Then Galσ (L/KL ) is solvable. Let M be σ ∗ -field extension of K. We say that difference equation (8.2.2) is solvable by elementary operations in M over K if M is a solution field for (8.2.2) over K and M is contained in a qLE of K. This concept is independent of the solution field M , as it follows from the second statement of the next theorem. Theorem 8.2.26 Let M be a σ ∗ -overfield of K. (i) Equation (8.2.2) is solvable by elementary operations in M over K if and only if the group Galσ (M/K) has a subnormal series whose factors are either finite or commutative. (ii) If (8.2.2) is solvable by elementary operations in M over K and N is another solution field for (8.2.2) over K, then (8.2.2) is solvable by elementary operations in N over K. (This property allows one to say that (8.2.2) is solvable by elementary operations over K if it is solvable by elementary operations in some solution field M ⊇ K.) (iii) If (8.2.2) is solvable by elementary operations over K and L a σ ∗ -overfield of K, then (8.2.2) is solvable by elementary operations over L.

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A number of results that specify the results of this section for the case of second order difference equations were obtained in [67] and [68]. C. Franke [67] also showed that the properties of having algebraically closed field of constants and having full sets of solutions of difference equations can be incompatible. Indeed, if K is a difference field with a basic set σ = {α} (Char K = 2) such that CK is algebraically closed, then the difference equation αy + y = 0 has no nonzero solution in K. (If b is such a solution, then α(b2 ) = b2 , so b2 ∈ CK . Since CK is algebraically closed, b ∈ CK contradicting the fact that α(b) = −b.) This observation and the fact that one could not associate a Picard-Vessiottype extension to every difference equations have led to a different approach to the Galois theory of difference equations. This approach, based on the study of simple difference rings rather than difference fields, was realized by M. van der Put and M. F. Singer in their monograph [159]. In the next section we give an outline of the fundamentals of the corresponding theory. We conclude this section with one more theorem on Galois correspondence for difference fields (see Theorem 8.2.30 below). This result is due to R. Infante who developed the theory of strongly normal difference field extensions (see [86] - [90]). Under some natural assumptions, the class of such extensions of a difference field K includes, in particular, the class of solution fields of linear homogeneous difference equations over K. We refer the reader to works [87] and [89] for the proofs. Let K be an ordinary inversive difference field of zero characteristic with a basic set σ = {α}. Let M be a finitely generated σ ∗ -overfield of K such that K is algebraically closed in M and CM = CK = C. As above, KM will denote the algebraic closure of K(CM ) in M . Furthermore, for any σ-isomorphism φ of M/K into a σ ∗ -overfield of M , Cφ will denote the field of constants of M φM ∗ . Definition 8.2.27 With the above conventions, M is said to be a strongly normal extension of K if for every σ-isomorphism φ of M/K into a σ ∗ -overfield of M , M Cφ ∗ = M φM ∗ = φM Cφ ∗ . Proposition 8.2.28 Let M be a solution field of a difference equation of the form (8.2.2) over K. Then M is a strongly normal extension of KM . Proposition 8.2.29 Let M be a strongly normal σ ∗ -field extension of K. Then (i) ld (M/K) = 1. (ii) If φ is any σ-isomorphism of M/K into a σ ∗ -overfield of M , then Cφ is a finitely generated extension of C and trdegM M φM ∗ = trdegC Cφ . Theorem 8.2.30 If M is a strongly normal σ ∗ -field extension of K, then there is a connected algebraic group G defined over CM such that the connected algebraic subgroups of G are in one-to-one correspondence with the intermediate σ ∗ -fields of M/K algebraically closed in M . Furthermore, there is a field of constants C  such that C  -rational points of G are all the σ-isomorphisms of M/K into M C  ∗ and this set is dense in G.

486 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY

8.3

Picard-Vessiot Rings and the Galois Theory of Difference Equations

In this section we discuss some basic results of the Galois theory of difference equations based on the study of simple difference rings associated with such equations. The complete theory is presented in [159] where one can find the proofs of all statements of this section. All difference rings and fields considered below are supposed to be ordinary and inversive. The basic set of a difference ring will be always denoted by σ and the only element of σ will be denoted by φ (we follow the notation of [159]). As usual, GLn (R) will denote the set of all nonsingular n × n-matrices over a ring R. Let R be a difference ring, A ∈ GLn (R), and Y a column vector (y1 , . . . , yn )T whose coordinates are σ ∗ -indeterminates over R. (The corresponding ring of σ ∗ -polynomials is still denoted by R{y1 , . . . , yn }∗ .) In what follows, we will study systems of difference equations of the form φY = AY where φY = (φy1 , . . . , φyn )T . Notice that an nth order linear difference equation φn y + · · · + a1 φy + a0 y = 0 (a0 , a1 , · · · ∈ R and y is a σ ∗ -indeterminate over R) is equivalent to such a system with yi = φi−1 y (i = 1, . . . , n) and ⎞ ⎛ 0 1 0 ... 0 ⎜ 0 0 1 ... 0 ⎟ ⎟. A=⎜ ⎝... ... ... ... ... ⎠ −a0 −a1 −a2 . . . −an−1 Clearly, A ∈ GLn (R) if and only if a0 = 0. With the above notation, a fundamental matrix with entries in R for φY = AY is a matrix U ∈ GLn (R) such that φU = AU (φU is the matrix obtained by applying φ to every entry of U ). If U and V are fundamental matrices for φY = AY , then V = U M for some M ∈ GLn (CR ) since U −1 V is left fixed by φ. (As in section 8.2, CR denotes the ring of constants of R, that is, CR = {a ∈ R|φa = a}.) Definition 8.3.1 Let K be a difference field. A K-algebra R is called a PicardVessiot ring (PVR) for an equation φY = AY

(A ∈ GLn (K))

(8.3.1)

if it satisfies the following conditions. (i) R is a σ ∗ -K-algebra (as usual, the automorphism of R which extends φ is denoted by the same letter). (ii) R is a simple difference ring, that is, the only difference ideals of R are (0) and R. (iii) There exists a fundamental matrix X for φY = AY having entries in R such that R = K[X, (det X)−1 ].

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487

The following example is due to M. Singer and M. van der Put (see [159, Examples 1.3 and 1.6]). Example 8.3.2 Let C be an algebraically closed field, Char C = 2. Let us define an equivalence relation on the set of all sequences a = (a0 , a1 , . . . ) of elements of C by saying that a is equivalent to b = (b0 , b1 , . . . ) if there exists N ∈ N such that an = bn for all n > N . With the coordinatewise addition and multiplication, the set of all equivalence classes forms a ring S. This ring can be treated as a difference ring with respect to its automorphism φ that maps an equivalent class of (a0 , a1 , . . . ) to the equivalent class of (a1 , a2 , . . . ). (It is easy to check that φ is well-defined.) To simplify notation we shall identify a sequence a with its equivalence class. Let R be the difference subring of S generated by C and j = (1, −1, 1, −1, . . . ), that is, R = C[j]∗ . The 1 × 1-matrix whose only entry is j is the fundamental matrix of the equation φy = −y. This ring is isomorphic to C[X]/(X 2 −1) (C[X] is a polynomial ring in one indetrminate X over C) whose only non-trivial ideals are generated by the cosets of X − 1 and X + 1. Since the ideals generated in R by j + 1 and j − 1 are not difference ideals, R is a simple difference ring. Therefore, R is a PVR for φy = −y over C. Note that R is reduced but it is not an integral domain. Proposition 8.3.3 Let K be a difference (σ ∗ -) field with an algebraically closed field of constants CK . (i) If a σ ∗ -K-algebra R is a simple difference ring finitely generated as a K-algebra, then CR = CK . (ii) If R1 and R2 are two PVR for a difference equation (8.3.1), then there exists a σ-isomorphism between R1 and R2 that leaves the field K fixed. To form a PVR for a difference equation (8.3.1) one can use the following procedure suggested in [159, Chapter 1]. Let (Xij ) denote an n × n-matrix of indeterminates over K and det denote the determinate of this matrix. Then 1 ] (we write one can extend φ to an automorphism of the K-algebra K[Xij , det 1 for det−1 ) by setting (φXij ) = A(Xij ). If I is a maximal difference ideal of det 1 1 ] then K[Xij , ]/I is a PVR for (8.3.1), it satisfies all conditions of K[Xij , det det 1 ]/I Definition 8.3.1. (It is easy to see that I is a radical σ ∗ -ideal and K[Xij , det is a reduced prime difference ring.) Moreover, any PVR for difference equation (8.3.1) will be of this form. 1 ]. Then Let K denote the algebraic closure of K and let D = K [Xij , det the automorphism φ extends to an automorphism of K which, in turn, extends to an automorphism of D such that (φXij ) = A(Xij ) (the extensions of φ are also denoted by φ). It is easy to see that every maximal ideal M of D has

488 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY the form (X11 − b11 , . . . , Xnn − bnn ) and corresponds to a matrix B = (bij ) ∈ GLn (K). Then φ(M ) is a maximal ideal of D that corresponds to the matrix A−1 φ(B) where φ(B) = (φ(bij )). Thus, the action of φ on D induces a map τ on GLn (K) such that τ (B) = A−1 φ(B). The elements f ∈ D are seen as functions on GLn (K). For any f ∈ D, B ∈ GLn (K), we have (φf )(τ (B)) = φ(f (B)). 1 ] such that φ(J) ⊆ J, then φ(J) = J. Furthermore, if J is an ideal of K[Xij , det Also, for reduced algebraic subsets Z of GLn (K), the condition τ (Z) ⊆ Z implies τ (Z) = Z. Proposition 8.3.4 ([159, Lemma 1.10]). The ideal J of a reduced subset Z of GLn (K) satisfies φ(J) = J if and only if Z(K) satisfies τ Z(K) = Z(K). An ideal I maximal among the φ-invariant ideals corresponds to a minimal (reduced) algebraic subset Z of GLn (K) such that τ Z(K) = Z(K). Such a set is called a minimal τ -invariant reduced set. Let Z be a minimal τ -invariant reduced subset of GLn (K) with an ideal 1 1 ] and let O(Z) = K[Xij , ]/I. Let us denote the image of Xij I ⊆ K[Xij , det det in O(Z) by xij and consider the rings     1 1 ⊆ O(Z) K Xij , K Xij , det det(Xij ) K     1 1 ⊇ C Yij , (8.3.2) C Yij , = O(Z) det(Yij ) det(Yij ) C

 1 Let (I) denote the ideal of O(Z) K K[Xij , ] generated by I and let J = det  1 ]. The ideal (I) is φ-invariant, the set of constants of O(Z) is C, (I) C[Yij , det  1 ]. Furthermore, one has and J generates the ideal (I) in O(Z) K K[Xij , det natural mappings O(Z) O(Z) → O(Z) K

 C Yij , = O(Z) C

    1 1 /J ← C Yij , /J . (8.3.3) det(Yij ) det(Yij )

Suppose that O(Z) is a separable extension of K (for example,  Char K = 0 or K is perfect). One can show (see [159, Section 1.2]) that O(Z) K O(Z) is reduced. 1 ]/J is reduced and J is a radical ideal. Furthermore, Therefore, C[Yij , det(Yij ) the following considerations imply that J is the ideal of an algebraic subgroup of GLn (C). Let A ∈ GLn (C) and let δA denote the action on the terms of (8.3.2) defined by (δA Xij ) = (Xij )A and (δA Yij ) = (Yij )A. Then the following eight properties are equivalent:

8.3. PICARD-VESSIOT RINGS AND THE GALOIS THEORY

489

(1) ZA = Z;  (2) ZA Z = ∅; (3) δA I = I; (4) I + δA I is not the unit ideal of K[Xij ,

1 ]; det

(5) δA (I) = (I); (6) (I) + δA (I) is not the unit ideal of O(Z) (7) δA J = J; (8) J + δA J is not the unit ideal of O(Z)





1 ]; det

K[Xij ,

C[Yij ,

1 ]. det

The set of all matrices A ∈ GLn (C) satisfying the equivalent conditions (1) - (8) form a group. Proposition 8.3.5 (see [159, Lemma 1.12]). Let O(Z) be a separable extension of K. With the above notation, A satisfies the equivalent conditions (1) - (8) if and only if A lies in the reduced subspace V of GLn (C) defined by J. Therefore, the set of such A is an algebraic group. Let G denote the group of all automorphisms of O(Z) over K which commute with the action of φ. The group G is called the difference Galois group of the equation φ(Y ) = AY over the field K. If δ ∈ G, then (δxij ) = (xij )A where A ∈ GLn (C) is such that δA (as defined above) satisfies δA I = I. Therefore, one can identify G and the subspace V 1 ]/J by O(G) and setting in the last proposition. Denoting the ring C[Yij , det  O(Gk ) = O(G) C k, GK = spec(O(Gk )), one can use (8.3.3) to obtain the sequence O(Z) → O(Z) O(Z) = O(Z) O(G) = O(Z) O(GK ) (8.3.4) K

C

K

The first embedding of rings corresponds to the morphism Z × GK → Z given by (z, g) → zg. The identification O(Z) O(Z) = O(Z) O(G) = O(Z) O(GK ) K

C

K

corresponds to the fact that the morphism Z × GK → Z × Z given by (z, g) → (zg, z) is an isomorphism. Thus, Z is a K-homogeneous space for GK , that is Z/K is a G-torsor. The following result (proved in [159, Section 1.2]) shows that a PVR is the coordinate ring of a torsor for its difference Galois group. Theorem 8.3.6 Let R be a separable PVR over K, a difference field with an algebraically closed field of constants C. Let G denote the group of the K-algebra automorphisms of R which commute with φ. Then (i) G has a natural structure as reduced linear algebraic group over C and the affine scheme Z over K has the structure of a G-torsor over K.

490 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY (ii) The set of G-invariant elements of R is K and R has no proper, nontrivial G-invariant ideals. (iii) There exist idempotents e0 , . . . , et−1 ∈ R (t ≥ 1) such that   a) R = R0 · · · Rt−1 where Ri = ei R for i = 0, . . . , t − 1. b) φ(ei ) = ei+1 (mod t) and so φ maps Ri isomorphically onto Ri+1 (mod t) and φt leaves each Ri invariant. c) For each i, Ri is a domain and is a Picard-Vessiot extension of ei K with respect to φt . Let K be a difference field with an algebraically closed field of constants C and R a PVR  for an equation φ(Y ) = AY over K. Let δ = δA and let R = R0 · · · Rt−1 (Ri = ei R for i = 0, . . . , t − 1) be as in the last theorem. Then δ : Ri → Ri+1 is an isomorphism and R0 is a PVR over K with respect to the automorphism δ t . Let us define two mappings Γ : Gal(R0 /K) → Gal(R/K)

∆ : Gal(R/K) → Z/tZ

and

as follows. For any ψ ∈ Gal(R0 /K), let Γ(ψ) = χ where for r = (r0 , . . . , rt−1 ) ∈ R, χ(r0 , . . . , rt−1 ) = (ψ(r0 ), δψδ −1 (r1 ), . . . , δ t−1 ψδ 1−t (rt−1 )). In order to define ∆, notice that if χ ∈ Gal(R/K), then χ permutes with each ei . If χ(e0 ) = ej , we define ∆(χ) = j. Proposition 8.3.7 Let R be a separable PVR over K, a difference field with an algebraically closed field of constants C. (i) With the above notation, we have the exact sequence Γ

∆

→ Gal(R/K) −→ Z/tZ → 0. 0 → Gal(R0 /K) − (ii) Let G denote the difference Galois group of R over K. If H 1 (Gal(K/K), 0, then Z = spec(R) is G-isomorphic to the G-torsor GK and so G(K)) =  R = C[G] K. In what follows we consider a characterization of the difference Galois group of a PVR over the field of rational functions C(z) in one variable z over an algebraically closed field C of zero characteristic (one can assume C = C). We fix a ∈ C(z), a = 0 and consider C(z) as an ordinary difference field with the basic automorphism φa : z → z + a (φa leaves the field C fixed). This difference field will be denoted by K. Note that φa does not extend to any proper finite field extension of K (see [159, Lemma 1.19]). Theorem 8.3.8 Let K = C(z) be as above and let G be an algebraic subgroup of GLn (C). Let φ(Y ) = AY be a system of difference equations with A ∈ G(K). Then (i) The Galois group of φ(Y ) = AY over K is subgroup of GC . (ii) Any minimal element in the set of C-subgroups H of G for which there exists a B ∈ Gln (K) with B −1 A−1 φ(B) ∈ H(K) is the difference Galois group of φ(Y ) = AY over K.

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491

(iii) The difference Galois group of φ(Y ) = AY over K is G if and only if for / any B ∈ G(K) and any proper C-subgroup H of G, one has B −1 A−1 φ(B) ∈ H(K). Definition 8.3.9 Let K be an ordinary difference field with a basic set σ = {φ} and let A ∈ Gln (K). A difference overring L of K is said to be the total PicardVessiot ring (TPVR) of the equation φ(Y ) = AY over K if L is the total ring of fractions of the PVR R of the equation.   As we have seen, a PVR R is a direct sum of domains: R = R0 · · · Rt−1 where each Ri is invariant under the action of φt . The automorphism φ of R permutes R0 , . . . , Rt−1 in a cyclic way (that is, φ(Ri ) = Ri+1 for i = 1, . . . , t − 2 = R0 ). It follows that the TPVR L is the direct sum of fields: and φ(R t−1 )  L = L0 · · · Lt−1 where each Li is the field of fractions of Ri , and φ permutes L0 , . . . , Lt−1 in a cyclic way. Proposition 8.3.10 With the above notation, let K be a perfect difference field with an algebraically closed field of constants C and let φ(Y ) = BY be a difference equation over K (B ∈ Gln (K)). Let a difference ring extension K  ⊇ K have the following properties: (i) The ring K  has no nilpotent elements and every non-zero divisor of K  is invertible. (ii) The set of constants of K  is C. (iii) There is a fundamental matrix F for the equation with entries in K  . (iv) K  is minimal with respect to (i), (ii), and (iii). Then K  is K-isomorphic as a difference ring to the TPVR of the equation. Corollary 8.3.11 Let K be as in the last proposition and let φ(Y ) = AY be a difference equation over K (A ∈ Gln (K)). Let a difference overring R ⊆ K have the following properties. (i) R has no nilpotent elements. (ii) The set of constants of the total quotient ring of R is C. (iii) There is a fundamental matrix F for the equation with entries in R. (iv) R is minimal with respect to (i), (ii), and (iii). Then R is a PVR of the equation.   With the above notation, let R = R0 · · · Rt−1 be the PVR of the equation φ(Y ) = AY (A ∈ Gln (K)). Let us consider the difference field (K, φt ) (that is, the field K treated as a difference field with the basic set σt = {φt }) and the difference equation φt (Y ) = At Y with At = φt−1 (A) . . . φ2 (A)φ(A)A. Proposition 8.3.12 (i) Each component Ri of R is a PVR for the equation φt (Y ) = At Y over the difference field (K, φt ). (ii) Let d ≥ 1 be a divisor of t. Using cyclic notation for the indices (t/d)−1 5   Ri+md of R = R0 · · · Rt−1 . {0, . . . , t − 1}, we consider subrings m=0

492 CHAPTER 8. ELEMENTS OF THE DIFFERENCE GALOIS THEORY Then each of these subrings is a PVR for the equation φd (Y ) = Ad Y over the difference field (K, φd ). Proposition 8.3.13 Let L be the TPVR of the equation φ(Y ) = AY (A ∈ Gln (K)) over a perfect difference field K whose field of constants C = CK is algebraically closed. Let G denote the difference Galois group of the equation and let H be an algebraic subgroup of G. Then G acts on L and moreover: (i) LG , the set of G-invariant elements of L, is equal to K. (ii) If LH = K, then H = G. The following result describes the Galois correspondence for total PicardVessiot rings. As it is noticed in [159, Section 1.3], one cannot expect a similar theorem for Picard-Vessiot rings. Indeed, let K be as in the last proposition and R = K C C[G] where C[G] is the ring of regular functions on an algebraic group G defined over C. For an algebraic subgroup H of G, the ring of invariants RH is the ring of regular functions on (G/H)K . In some cases, e. g., G = Gln (C) and H a Borel subgroup, the space G/H is a connected projective variety and so the ring of regular functions on (G/H)K is just K. Theorem 8.3.14 Let K be a difference field of zero characteristic with a basic set σ = {φ}. Let A ∈ GLn (K) and let L be a TPVR of the equation φ(Y ) = AY over K. Let F denote the set of intermediate difference rings F such that K ⊆ F ⊆ L and every non-zero divisor of F is a unit of F . Furthermore, let G denote the set of algebraic subgroups of G. (i) For any F ∈ F, the subgroup G(L/F ) ⊆ G of the elements of G which fix F pointwise, is an algebraic subgroup of G. (ii) For any algebraic subgroup H of G, the ring LH belongs to F. (iii) Let α : F → G and β : G → F denote the maps F → G(L/F ) and H → LH , respectively. Then α and β are each other’s inverses. Corollary 8.3.15 With the notation of Theorem 8.3.14, a group H ∈ G is a normal subgroup of G if and only if the difference ring F = LH has the property that for every z ∈ F \ K, there is an automorphism δ of F/K which commutes with φ and satisfies δz = z. If H ∈ G is normal, then the group of all automorphisms δ of F/K which commute with φ is isomorphic to G/H. Corollary 8.3.16 With the above notation, suppose that an algebraic group H ⊆ G contains G0 , the component of the identity of G. Then the difference ring RH (R is a PVR for the equation φ(Y ) = AY over K) is a finite dimension vector space over K with dimension equal to G : H. A number of applications of the above-mentioned results on ring-theoretical difference Galois theory to various types of algebraic difference equations can be found in [78] - [81] and [159]. Using the technique of difference Galois groups, the monograph [159] also develops the analytic theory of ordinary difference equations over the fields C(z) and C({z −1 }). We conclude this section with a fundamental result on the inverse problem of the ring-theoretical difference Galois theory.

8.3. PICARD-VESSIOT RINGS AND THE GALOIS THEORY

493

Theorem 8.3.17 ([159, Theorem 3.1]). Let K = C(z) be the field of fractions of one variable z over an algebraically closed field C of zero characteristic. Consider K as an ordinary difference field with respect to the automorphism φ that leaves the field C fixed and maps z to z + 1. Then any connected algebraic subgroup G of Gln (C) is the difference Galois group of a difference equation φ(Y ) = AY , A ∈ Gln (K).

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Index A Abelian category, 10–12 Additive category, 9 functor, 9 Adhered function, 429, 431 Admissible order, 96 transformation, 202–204, 206–209 Affine K-algebra, 35 algebraic K-variety, 30, 31 A-leader of a difference (σ-) polynomial, 130 Algebra of difference polynomials, 117 of inversive difference polynomials, 118 of σ-polynomials, 116–119 of σ ∗ -polynomials, 116–118 Algebraic element, 65 field extension, 65–69 closure, 66–69 Algebraically closed field, 67 dependent element, 76 set, 76 disjoint fields, 76, 79 independent element, 76 independent set, 76 Almost every specialization, 86, 87 α-derivation, 94, 95 α-invariant element, 114 αi -periodic, 114 Antisymmetric relation, 3 Amonadic difference (σ-) field extension, 354–356 Artinian module, 22–23 ring, 22–23 Aperiodic difference (σ-) ring, 114 Ascending chain condition, 22 Associated graded module, 44

object, 11 ring, 44 prime ideals, 25 primes belonging to an ideal, 26 Automorphism, 12 Autoreduced set, 130–132, 134, 135, 138, 270 (<1 , . . . ,