Abelian varieties over fields of finite characteristic

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1 Cent. Eur. J. Math. 125) DOI: /s Central European Journal of Mathematics Abelian varieties over fields of finite characteristic Research Article Yuri G. Zarhin 1,2 1 Department of Mathematics, Pennsylvania State University, University Park, PA 16802, USA 2 Department of Mathematics, The Weizmann Institute of Science, P.O.B. 26, Rehovot , Israel Received 3 February 2013; accepted 19 September 2013 Abstract: The aim of this paper is to extend our previous results about Galois action on the torsion points of abelian varieties to the case of finitely generated) fields of characteristic 2. MSC: 11G10, 14K15 Keywords: Abelian varieties Isogenies Points of finite order Tate modules Versita Sp. z o.o. 1. Introduction Let K be a field, K its algebraic closure, K s K the separable algebraic closure of K, GalK) = GalK s /K) = AutK/K) the absolute Galois group of K. If l is a prime then we write F l for the finite field Z/lZ. Let X be an abelian variety over K. Then we write EndX) for its ring of K-endomorphisms and End 0 X) for the finite-dimensional semisimple Q-algebra EndX) Q. If n is a positive integer that is not divisible by chark) then we write X n for the kernel of multiplication by n in XK); it is well known that X n is free Z/nZ-module of rank 2 dim X [9], which is a Galois submodule of XK s ). We write ρ n,x for the corresponding continuous) structure homomorphism ρ n,x : GalK) Aut Z/nZ X n ) = GL2 dim X, Z/nZ). In particular, if n = l is a prime different from chark) then X l is a 2 dim X-dimensional F l -vector space provided with ρ l,x : GalK) Aut Fl X l ) = GL2 dim X, F l ). zarhin@math.psu.edu 659

2 Abelian varieties over fields of finite characteristic We write G l = G l,x,k for the image subgroup) ρ l,x GalK)) Aut Fl X l ). By definition, Gl,X,K is a finite subgroup of Aut Fl X l ) = GL2 dim X, F l ). If KX l ) is the field of definition of all points of order l on X then it is a finite Galois extension of K and the corresponding Galois group GalKX l )/K) is canonically isomorphic to G l,x,k. If K /K is a finite Galois extension of fields then GalK ) is a normal open subgroup of finite index in GalK) while X = X K K is a dim X-dimensional abelian variety over K and the GalK )-modules X l and X l are canonically isomorphic. Under this isomorphism, G l,x,k becomes isomorphic to a certain normal subgroup of G l,x,k. By functoriality, EndX) acts on X n. This action gives rise to the embedding of free Z/nZ-modules EndX) Z/nZ End Z/nZ X n ); in addition, the image of EndX) Z/nZ lies in the centralizer End GalK) X n ) of GalK) in End Z/nZ X n ). Further we will identify EndX) Z/nZ with its image in End GalK) X n ) and write EndX) Z/nZ End GalK) X n ). If l is a prime that is different from chark) then we write T l X) for the Z l -Tate module of X and V l X) for the corresponding Q l -vector space T l X) Zl Q l provided with the natural continuous Galois action [14] ρ l,x : GalK) Aut Zl T l X)) Aut Ql V l X)). Recall [9] that T l X) is a free Z l -module of rank 2 dim X and V l X) is a Q l -vector space of dimension 2 dim X. Notice that there are canonical isomorphisms of GalK)-modules X l i = T l X)/l i T l X) for all positive integers i. The natural embeddings EndX) Z/l i Z End Z/l i ZX l i) are compatible and give rise to the embeddings of Z l -algebras EndX) Z l End Zl T l X)) and Q l -algebras EndX) Q l End Ql V l X)). Again, the images of EndX) Z l in End Zl T l X)) and of EndX) Q l in End Ql V l X)) lie in the centralizers End GalK) T l X)) and End GalK) V l X)) respectively. We will identify EndX) Z l and EndX) Q l with their respective images and write EndX) Z l End GalK) T l X)) End Zl T l X)), EndX) Q l End GalK) V l X)) End Ql V l X)). Similarly, if Y is another abelian variety over K then we write HomX, Y ) for the free commutative) group of all K-homomorphisms from X to Y. Similarly, there are the natural embeddings Let HomX, Y ) Z/nZ Hom GalK) X n, Y n ) Hom Z/nZ X n, Y n ), HomX, Y ) Z/l i Z Hom GalK) X l i, Y l i) Hom Z/l i ZX l i, Y l i), HomX, Y ) Z l Hom GalK) T l X), T l Y )) Hom Zl T l X), T l Y )), HomX, Y ) Q l Hom GalK) V l X), V l Y )) Hom Ql V l X), V l Y )). ρ l,x : GalK) Aut Zl T l X)) Aut Ql V l X)) be the corresponding l-adic representation of GalK). The image is a compact l-adic Lie sub)group [11, 14]. G l,x,k = ρ l,x GalK)) Aut Zl T l X)) Aut Ql V l X)) Let d be a positive integer. We write IsogX, K, d) for the set of K-isomorphism classes of abelian varieties Y over K that enjoy the following properties: 660

3 Yu.G. Zarhin i) Y admits a K-polarization of degree d. ii) There exists a K-isogeny Y X whose degree is prime to chark). For example, if d = 1 then IsogX, K, 1) consists of K-isomorphism classes of) all principally polarized abelian varieties Y over K that admit a K-isogeny whose degree is prime to chark). We write IsogX, K) for the set of K-isomorphism classes of abelian varieties Y over K such that there exists a K- isogeny Y X whose degree is prime to chark). Clearly, IsogX, K) coincides with the union of all IsogX, K, d). The following statement was proven under an additional assumption that p does not divide d by the author when p > 2 [18] and by Mori when p = 2 [8, Chapter XII, Corollary 2.4, p. 244]. This is a strengthening of Tate finiteness conjecture for isogenies of abelian varieties [16, 29].) Theorem 1.1 [8, Corollary 2.4, p. 244]). Assume that p def = chark) > 0 and K is finitely generated over the finite prime field F p. Let d be a positive integer and X be an abelian variety over K. Then the set IsogX, K, d) is finite. The finiteness of IsogX, K, d) combined with results of [17] implies the Tate conjecture on homomorphisms of abelian varieties and the semisimplicity of Tate modules over K see [18] for p > 2 and [8, Chapter XII, Theorem 2.5, pp ]). Theorem 1.2 [8, Theorem 2.5, pp ]). Assume that p def = chark) > 0 and K is finitely generated over the finite prime field F p. Then for all abelian varieties A and B over K and every prime l chark) the Galois module V l A) is semisimple and the natural embedding of Z l -modules HomA, B) Z l Hom GalK) T l A), T l B)) is bijective. Remark 1.3. In fact, Theorem 1.2 follows even from a special case of Theorem 1.1 that deals only with principally polarized abelian varieties i.e., when d = 1), see Remark By [16, Section 1, Lemma 1] the second assertion of Theorem 1.2 implies the following statement. Theorem 1.4. Assume that p def = chark) > 0 and K is finitely generated over the finite prime field F p. Then for all abelian varieties A and B over K the natural embedding of Q l -vector spaces HomA, B) Q l Hom GalK) V l A), V l B)) is bijective Let K be a field that is finitely generated over the finite prime field F p and A an abelian variety of positive dimension over K. Let l be a prime different from p. By Theorem 1.4 applied to B = A) and Theorem 1.2, the GalK)-module V l A) is semisimple and End GalK) V l A)) = EndA) Q l = End 0 A) Q Q l. Since G l,a,k is the image of GalK) Aut Zl T l A)) Aut Ql V l A)), the G l,a,k -module V l A) is semisimple and End Gl,A,K V l A)) = EndA) Q l = End 0 A) Q Q l. 661

4 Abelian varieties over fields of finite characteristic Let Q l G l,a,k be the Q l -subalgebra of End Ql V l A)) spanned by G l,a,k. It follows from the Jacobson density theorem [6, Chapter XVII, Section 3, Theorem 1] that Q l G l,a,k coincides with the centralizer of EndA) Q l in End Ql V l A)). It follows easily that if Z l G l,a,k is the Z l -subalgebra of End Zl T l A)) spanned by G l,a,k then the centralizer of EndA) Z l in End Zl T l A)) contains Z l G l,a,k as a Z l -submodule of finite index. 2. Main results The aim of this note is to prove variants of Theorem 1.2 where Tate modules are replaced by Galois modules A n and B n. Most of our results were already proven in [19, 28] or stated in [21] under an additional assumption p > 2. See also [27] where the case of finite fields is discussed.) Throughout this section, K is a field that is finitely generated over the finite prime field F p. Theorem 2.1. Let A be an abelian variety of positive dimension over K. Then the set IsogA, K) is finite. Remark 2.2. A weaker version of Theorem 2.1 where IsogA, K) is replaced by its subset that consists of abelian varieties that admit a polarization of degree prime to p) is proven in [19, Theorem 6.1] under an additional assumption that p > 2. Corollary 2.3. Let A be an abelian variety of positive dimension over K. There exists a positive integer r = ra) that is not divisible by p and enjoys the following properties. i) If C is an abelian variety over K that admits a K-isogeny C A whose degree is not divisible by p then there exists a K-isogeny β : A C with ker β A r. ii) If n is a positive integer that is not divisible by p and W is a Galois submodule in A n then there exists u EndA) such that rw ua n ) W. iii) For all but finitely many primes l the Galois module A l is semisimple. Remark 2.4. Corollary 2.3 iii) is proven in [19, Theorem 1.1] under an additional assumption p > 2. Theorem 2.5. Let A be an abelian variety over K. Then there exists a positive integer r 1 = r 1 A) such that, for any positive integer n that is not divisible by p and u n an endomorphism of the Galois module A n, it enjoys the following property: If we put m = n/n, r 1 ) then there exists u EndA) such that both u and u n induce the same endomorphism of the Galois module A m. If A and B are abelian varieties over K then applying Theorem 2.5 to their product X = A B, we obtain the following statement. Theorem 2.6. Let A and B be abelian varieties over K. Then there exists a positive integer r 2 = r 2 A, B) such that, for any positive integer n that is not divisible by p and u n : A n B n a homomorphism of the Galois modules, it enjoys the following property: If we put m = n/n, r 1 ) then there exists u HomA, B) such that both u and u n induce the same homomorphism of the Galois modules A m B m. Theorem 2.6 implies readily the following assertion. 662

5 Yu.G. Zarhin Corollary 2.7. Let A and B be abelian varieties over K. Then for all but finitely many primes l the natural injection HomA, B) Z/lZ Hom GalK) A l, B l ) is bijective. Remark 2.8. Theorem 2.6 was stated without proof in [21] under an additional condition that p > 2. Corollary 2.7 was proven in [19, Theorem 1.1] under an additional condition that p > 2. Theorem 2.9. Let A be an abelian variety of positive dimension over K. Then for all but finitely many primes l the centralizer of EndA) Z l in End Zl T l A)) coincides with Z l G l,a,k. Remark When K is a field of characteristic zero that is finitely generated over Q, an analogue of Theorem 2.9 was proven by Faltings [3, Section 3, Theorem 1 c)]. Recall that an old result of Grothendieck [10] asserts that in characteristic p an abelian variety of CM-type is isogenous to an abelian variety that is defined over a finite field. The converse follows from a theorem of Tate [16].) Theorem Let X be an abelian variety over K. Suppose that for infinitely many primes l the group G l,x,k is commutative. Then X is an abelian variety of CM-type over K and therefore is isogenous over K to an abelian variety that is defined over a finite field. Theorem 2.11 may be strengthened as follows. Theorem Let X be an abelian variety over K. Suppose that for infinitely many primes l the group G l,x,k is l-solvable, i.e., its Jordan Hölder factors are either l-groups or groups whose order is not divisible by l. Then there is a finite Galois extension K /K such that X K K is an abelian variety of CM-type over K and therefore is isogenous over K to an abelian variety that is defined over a finite field. Theorem 2.12 combined with the celebrated theorem of Feit Thompson about solvability of groups of odd order) implies readily the following statement. Corollary Let X be an abelian variety of positive dimension over K that is not isogenous over K to an abelian variety that is defined over a finite field. Then for all but finitely many primes l the group G l,x,k is not solvable and its order is divisible by 2l. Remark See [24 26] for plenty of explicit examples of abelian varieties in characteristic p without CM. Theorem 2.11 was proven in [20] under an additional assumption that p > 2. Theorem 2.12 was stated without proof in [21] under an additional assumption that K is global and p >

6 Abelian varieties over fields of finite characteristic In order to state another partial) strengthening of Theorem 2.11, we need to introduce the following notation. If A is a commutative group then we write TORSA) for its subgroup of all periodic elements and TORSA)non-p) that consists of all elements of TORSA), whose order is prime to p. Theorem Let K ab K s be the maximal abelian extension of K. Let X be a simple abelian variety over K. If TORSXK ab ))non-p) is infinite then X is an abelian variety of CM-type over K and therefore is isogenous over K to an abelian variety that is defined over a finite field. Remark In characteristic zero an analogue of Theorem 2.15 was proven in [23, Theorem 1.5]. Theorem 2.15 implies readily the following statement. Compare with [23, Corollary, p. 132].) Corollary Let K ab K s be the maximal abelian extension of K. Let X be an abelian variety of positive dimension over K. Let X 1,..., X r be simple abelian varieties over K such that the product r i=1 X i is K-isogenous to X. Then TORSXK ab ))non-p) is finite if and only if all the groups TORSX i K ab ))non-p) are finite, i.e., all X i are not of CM-type over K, 1 i r. Now we discuss the torsion of abelian varieties in infinite Galois extensions of K with finite field of constants. Theorem Let X be an abelian variety of positive dimension over K such that the center of End 0 X) is a direct sum of totally real number fields. Let K K s be an infinite Galois extension of K. Let F be the algebraic closure of F p in K and suppose that F is a finite field. Then TORSXK ))non-p) is finite. Theorem 2.18 is an immediate corollary of the conjunction of following two assertions. Theorem Let X be an abelian variety of positive dimension over K such that the center of End 0 X) is a direct sum of totally real number fields. Let K K s be an infinite Galois extension of K. If l p is a prime such that the l-primary component of TORSXK )) is infinite then K contains all l-power roots of unity. In particular, the algebraic closure of F p in K is infinite. Theorem Let X be an abelian variety of positive dimension over K such that the center of End 0 X) is a direct sum of totally real number fields. Let us choose a polarization λ: X X t that is defined over K. Let l p be a prime that enjoys the following properties: i) l is odd and prime to deg λ; ii) the GalK)-module X l is semisimple and End GalK) X l ) = EndX) Z/lZ. If the l-primary component of TORSXK )) does not vanish then K contains a primitive lth root of unity. In particular, if F be the algebraic closure of F p in K then its order is strictly greater than l. Remark Let S be the set of primes l that do not enjoy either property i) or property ii). Then S is finite. 664

7 Yu.G. Zarhin The paper is organized as follows. In Section 3 we discuss isogenies of abelian varieties and their kernels viewed as finite Galois modules). One of the goals of our approach is to stress the role of analogues of Tate s finiteness conjecture for isogeny classes of abelian varieties. In Section 4 we prove all the main results except Theorem 2.12, which will be proven in Section 5. Section 6 contains additional references to results that may be extended to characteristic 2 case. 3. Isogenies and finite Galois modules We write P for the set of all primes. Let K be a field. Let P P be a nonempty set of primes that does not contain chark). If X and Y are abelian varieties over K then a K-isogeny X Y is called a P-isogeny if all prime divisors of its degree are elements of P. For example, if P is a singleton {l} then a P-isogeny is nothing else but an l-power isogeny. We say that X and Y are P-isogenous over K if there is a P-isogeny X Y that is defined over K. The property to be P-isogenous is an equivalence relation. Indeed, one has only to check that there exists a P-isogeny v : Y X that is defined over K. Indeed, thanks to the Lagrange theorem, ker u X n where n def = deg u = #ker u). It follows that there is a K-isogeny v : Y X such that the composition vu: X Y X coincides with multiplication by n in X. This implies that n 2 dim X = degvu) = deg v deg u. Since u is a P-isogeny, all prime divisors of n belong to P. This implies that all prime divisors of deg v also belong to P, i.e., v is a P-isogeny and we are done. Let X t and Y t be the dual abelian varieties over K) of X and Y respectively and u t : Y t X t, v t : X t Y t be the K-isogenies that are duals of u and v respectively. Since deg u t = deg u, deg v t = deg v, X t and Y t are also P- isogenous over K. Warning: X and X t are not necessarily P-isogenous!) This implies that if X and Y are P-isogenous over K then X X t ) 4 and Y Y t ) 4 are also P-isogenous over K. We write Is P X, K) for the set of isomorphism classes of abelian varieties Y over K that are P-isogenous to X over K. We write Is P X, K, 1) for the subset of Is P X, K) that consists of all isomorphism classes of Y with principal polarization over K. For example, if P is P \ {chark)} then Is P X, K) = IsogX, K), Isog P X, K, 1) = IsogX, K, 1). Now Theorem 2.1 becomes an immediate corollary of Theorem 1.1 and the following statement. Theorem 3.1. Let X be an abelian variety over a field K. Suppose that the set Is P X X t ) 4, K, 1 ) is finite. Then the set Is P X, K) is also finite. Proof. Let us fix an abelian variety X be over K. Let Y be an abelian variety over K that is P-isogenous to X over K. As we have seen, Y Y t ) 4 is P-isogenous to X X t ) 4 over K. Recall [8, 19, 22] see also [27, Section 7]) that Y Y t ) 4 admits a principal polarization over K 1. Since the set Is P Y Y t ) 4, K, 1 ) is finite, the set of K-isomorphism classes of all Y Y t ) 4 with fixed X) is finite. On the other hand, each Y is isomorphic to a K-abelian subvariety of Y Y t ) 4 over K. But the set of isomorphism classes of abelian subvarieties of a given abelian variety is finite [7]. This implies that the set of K-isomorphism classes of all Y is finite. 1 In [8, Chapter IX, Section 1] Deligne s proof is given. 665

8 Abelian varieties over fields of finite characteristic Corollary 3.2. Let X be an abelian variety of positive dimension over a field K. Suppose that the set Is P X X t ) 4, K, 1 ) is finite. Then there exists a positive integer r = rx) that is not divisible by chark) and enjoys the following properties. i) If Y is an abelian variety over K that is P-isogenous to X over K then there exists a P-isogeny β : X Y over K with ker β X r. ii) If n is a positive integer, all whose prime divisors lie in P and W is a Galois submodule in X n, then there exists u EndX) such that rw ux n ) W. Proof. By Theorem 3.1, there are finitely many K-abelian varieties Y 1,..., Y d that are P-isogenous to X over K and such that every K-abelian variety Y that is P-isogenous to X over K is K-isomorphic to one of Y j. For each Y i pick a P-isogeny β i : X Y i that is defined over K. Clearly, ker β i X mi where m i = deg β i. Let us put r = d i=1 m i. Clearly, for all Y i, ker β i X mi X r. This implies that for every K-abelian variety Y that is P-isogenous to X over K there exists a P-isogeny β : X Y over K whose kernel lies in X r. This proves i), since every prime divisor of r is a prime divisor of one of m i = deg β i and therefore lies in P. ii) The quotient Y = X/W is an abelian variety over K. The canonical map π : X X/W = Y is a P-isogeny over K, because deg π = #W ) divides #X n ) = n 2 dim X. This implies that Y is P-isogenous to X over K. The rest of the proof goes literally with the same notation) as in [27, Section 8, pp ] provided one replaces the reference to [27, Corollary 3.5 i)] by the already proven case i). In [27], n X : X X and n Y : Y Y denote the multiplication by n in X and Y respectively.) Theorem 3.3. Suppose that P is infinite. Let X be an abelian variety of positive dimension over a field K. Suppose that the set Is P X X t ) 4, K, 1 ) is finite. Then for all but finitely many primes l the Galois module X l enjoys the following property: If W is a Galois submodule in X l then there exists ũ EndX) Z/lZ such that ũ 2 = ũ and ũx n ) = W. In particular, the Galois module X n splits into a direct sum X n = ũx n ) 1 ũ)x n ) = W 1 ũ)x n ) of its Galois submodules W and 1 ũ)x n ). Theorem 3.3 implies immediately the following assertion. Corollary 3.4. Suppose that P is infinite. Let X be an abelian variety of positive dimension over a field K. Suppose that the set Is P X X t ) 4, K, 1 ) is finite. Then for all but finitely many primes l P the Galois module X l is semisimple. Proof of Theorem 3.3. It is well known that for all but finitely many primes l the finite-dimensional F l -algebra EndX) Z/lZ is semisimple. See, e.g., [19, Lemma 3.2].) Let r be as in Corollary 3.2. Now let l P be a prime that does not divide r and such that EndX) Z/lZ is semisimple. Let W be a Galois submodule in X l. By Corollary 3.2, there exists u EndX) such that rw ux l ) W. Since l does not divide r, we have rw = W and therefore ux l ) = W. Let u l be the image of u in EndX) Z/lZ. Clearly, u l X l ) = ux l ) = W. Let I be the right ideal in semisimple EndX) Z/lZ generated by u l. The semisimplicity implies that there exists an idempotent ũ that generates I. It follows that W = u l X l ) = ũx l ). 666

9 Yu.G. Zarhin We will need the following lemma [27, Lemma 9.2, p. 333]. Lemma 3.5. Let Y be an abelian variety of positive dimension over an arbitrary field K. Then there exists a positive integer h = hy, K) that enjoys the following property: If n is a positive integer that is not divisible by chark), u, v EndY ) are endomorphisms of Y such that {ker u Y n } {ker v Y n } then there exists a K-isogeny w : Y Y such that hv wu n EndY ). In particular, the images of hv and wu in coincide. EndY ) Z/nZ End GalK) Y n ) End Z/nZ Y n ) Theorem 3.6. Let X be an abelian variety of positive dimension over a field K. Suppose that the set Isog P X X t ) 8, K, 1 ) is finite. Then there exists a positive integer r 1 = r 1 X, K) such that, for any positive integer n, all whose prime divisors lie in P and m = n/n, r 1 ), if enjoys the following property: If u n End GalK) X n ) then there exists u EndX) such that the images of u n and u in End GalK) X m ) coincide. Proof. Let us put Y = X X. Then Y Y t ) 4 = X X t ) 8. Let ry ) be as in Corollary 3.2 and hy ) as in Lemma 3.5. Let us put r 1 = r 1 X, K) = ry, K)hY, K). Now the proof goes literally as the the proof of [27, Section 10, Theorem 4.1], provided one replaces the references to [27, Corollary 3.5, Lemma 9.2] by references to Corollary 3.2 and Lemma 3.5 respectively. Let A and B be abelian varieties over K. decompositions Applying Theorem 3.6 to X = A B and using the obvious compatible EndX) = EndA) EndB) HomA, B) HomB, A), End GalK) X n ) = End GalK) A n ) End GalK) B n ) Hom GalK) A n, B n ) Hom GalK) B n, A n ), we obtain the following statement. Theorem 3.7. Let A and B be abelian varieties of positive dimension over a field K. Suppose that the set Is P A B A t B t ) 8, K, 1 ) is finite. Then there exists a positive integer r 2 = r 2 A, B, K) = r 1 A B, K) such that, for any positive integer n, all whose prime divisors lie in P and m = n/n, r 2 ), it that enjoys the following property: If u n Hom GalK) A n, B n ) then there exists u HomA, B) such that the images of u n and u in Hom GalK) A m, B m ) coincide. Corollary 3.8. Let A and B be abelian varieties of positive dimension over a field K. Suppose that the set Is P A B A t B t ) 8, K, 1 ) is finite. Then for all primes l P the natural injection HomA, B) Z l Hom GalK) T l A), T l B)) is bijective. Proof. Let r 2 = r 2 A, B) be as in Theorem 3.7. Let l i 0 be the exact power of l that divides r 2. Let v Hom GalK) T l A), T l B)). For each i > i 0, v induces a homomorphism v i Hom GalK) A l i, B l i). By Theorem 3.7, there exists u i HomA, B) such that the images of u i and v i in HomA l i i 0, B l i i 0 ) coincide. This means that u i v sends T l A) into l i i 0T l B). It follows that v coincides with the limit of the sequence {u i } i>i 0 in Hom Zl T l A), T l B)) with respect to l-adic topology. Since HomA, B) Z l is a compact and therefore a closed subset of Hom Zl T l A), T l B)), the limit v also lies in HomA, B) Z l. 667

10 Abelian varieties over fields of finite characteristic The following lemma will be proven at the end of this section. Lemma 3.9. Let X be an abelian variety of positive dimension over a field K. Suppose that the set Isog P X X t ) 4, K, 1 ) is finite. Let r = rx, K) be as in Corollary 3.2. Then every l P enjoys the following property. Let S be a Galois-invariant Z l -submodule in T l X) such that the quotient T l X)/S is torsion-free. Then there exists u EndX) Z l such that r S ut l X)) S. Theorem Let X be an abelian variety of positive dimension over a field K. Suppose that the set Isog P X X t ) 8, K, 1 ) is finite. Then every l P enjoys the following property. If W is a GalK)-invariant Q l -vector subspace in V l X) then there exists ũ EndX) Q l such that ũ 2 = ũ and ũv l X)) = W. In particular, V l X) splits into a direct sum V l X) = ũv l X)) 1 ũ)v l X)) = W 1 ũ)v l X)) of its Galois submodules W and 1 ũ)v l X)) and the GalK)-module V l X) is semisimple. Proof. Let us put S = W T l X). Clearly, S is a Galois-invariant free Z l -submodule in T l X) and W = Q l S. In addition, the quotient T l X)/S is torsion-free. By Lemma 3.9, there exists u EndX) Z l such that r S ut l X)) S. It follows that r W uv l X)) W. Since r W = W, we have uv l X)) = W. Notice that EndX) Z l EndX) Q l = End 0 X) Q Q l and End 0 X) is a finite-dimensional semisimple Q-algebra. It follows that EndX) Q l is a finite-dimensional semisimple Q l -algebra. Let I the left ideal in semisimple EndX) Q l generated by u; there is an idempotent ũ that generates I. Clearly, ũv l X)) = uv l X)) = W. Proof of Lemma X of X. If S = {0} then we just put u = 0. If S = T l X) then we take as u the identity automorphism So, further we assume that S is a proper free Z l -module in T l X) of positive rank say, d and let {e 1,..., e j,..., e d } be its basis. Since S is pure in T l X), for all positive integers i the natural homomorphism of Galois modules S i def = S/l i S T l X)/l i T l X) = X l i is an injection of free Z/l i Z-modules. Further, we will identify S i with its image in X l i. We write ej i for the image of e j in S i X l i; clearly, the d-element set {ej i } d j=1 is a basis of the free Z/li Z-module S i. By Corollary 3.2 applied to n = l i and W = S i there exists u i EndX) such that rs i u i X l i) S i. In particular, u i X l i) contains rej i for all j. It is also clear that for each z T l X), u i z) S + l i T l X). For each ej i pick an element z i j T l X)/l i T l X) = X l i such that u i zj i ) = rej i. Let us pick zj i T l X) such that its image in X l i coincides with zj i. Clearly, the image of u i zj) i in T l X)/l i T l X) = X l i equals rej i. Using the compactness of EndX) Z l and T l X), let us choose an infinite increasing sequence of positive integers i 1 < i 2 <... < i m <... such that {u im } m=1 converges in EndX) Z l to some u and { j } z i converges in T m m=1 lx) to some z j for all j with 1 j d. It follows that uz j ) = lim u im z j i m ) = rej. This implies that ut l X)) r S. On the other hand, for each z T l X), u im z) S + l im T l X). Since {i m } m=1 is an increasing set of positive integers, {S + l im T l X)} m=1 is a decreasing set of compact sets whose intersection is compact S. It follows that uz) = lim u im z) lies in S. 668

11 Yu.G. Zarhin Lemma Let X be an abelian variety of positive dimension over K. Let l be a prime that is different from chark) and such that the GalK)-module X l is semisimple and End GalK) X l ) = EndX) Z/lZ. Then the centralizer of EndX) Z l in End Zl T l X)) coincides with Z l G l,x,k. Proof. and Clearly, G X,l,K is the image of GalK) G l,x,k Aut Fl X l ). It follows that the G X,l,K -module X l is semisimple End GX,l,K X l ) = EndX) Z/lZ. By the Jacobson density theorem, F l GX,l,K coincides with the centralizer of EndX) Z/lZ in End Fl X l ). Here F l GX,l,K is the F l -subalgebra of End Fl X l ) spanned by G X,l,K.) Let M be the centralizer of EndX) Z l in End Zl T l X)). Clearly, M is a saturated Z l -submodule of End Zl T l X)) i.e., the quotient End Zl T l X))/M is torsion-free); in addition, M contains Z l G l,x,k. We have M/lM End Zl T l X)) Z l /lz l = End Fl X l ). Clearly, M/lM lies in the centralizer of EndX) Z l Zl Z l /lz l = EndX) Z/lZ. This implies that M/lM F l GX,l,K End Fl X l ). On the other hand, the image of Z l G l,x,k in End Zl T l X)) Z l /lz l = End Fl X l ) obviously coincides with F l GX,l,K. Since this image lies in M/lM, we conclude that M/l = F l GX,l,K and M = Z l G l,x,k + l M. It follows from Nakayama s lemma that the Z l -module M coincides with its submodule Z l G l,x,k. Remark Let P be a singleton {l} and d = 1. Now Theorem 1.1 combined with Corollary 3.8 and Theorem 3.10 implies readily Theorem Proof of main results Throughout this section, K is a field that is finitely generated over F p. Let us put P = P \ {p}. Proof of Corollary 2.3 and Theorem 2.5. Corollary 2.3 follows readily from Corollary 3.2 combined with Theorem 1.1. Theorem 2.5 follows readily from Theorem 3.6 combined with Theorem 1.1. Proof of Theorems 2.1 and 2.6. One has only to combine Theorem 1.1 with Theorems 3.1 and 3.7 respectively. The assertion follows readily from Lemma 3.11 combined with Corollary 2.3 and Theo- Proof of Theorem 2.9. rem 2.5. Proof of Theorem The proof of [20, Theorem 4.7.4] works literally provided one replaces the reference to [19, Theorem 1.1.1] by references to Corollaries 2.3 and

12 Abelian varieties over fields of finite characteristic Proof of Theorem Theorem 2.15 is an immediate corollary of the conjunction of two following statements. Compare with [23, Theorems 2 and 3, p. 133].) Theorem 4.1. Let X be a simple abelian variety over K that is not of CM-type. Let l p be a prime, W a nonzero Galois-invariant Q l -vector space in V l X) and G W the image of GalK) in Aut Ql W ). Then the group G W is not commutative. Theorem 4.2. Let X be a simple abelian variety over K that is not of CM-type. Then for all but finitely many primes l p the following condition holds: Let W a nonzero Galois-invariant F l -vector space in X l and G W the image of GalK) in Aut Fl W ), then the group G W is not commutative. Proof of Theorems 4.1 and 4.2. The proof of [23, Section 3, Theorems 2 and 3] works literally provided one replaces the reference to [23, p. 139, Statements 1 and 2] by references to Theorem 1.2 instead of Statement 1) and to Corollaries 2.3 and 2.7 instead of Statement 2). This ends the proof of Theorem Proof of Theorems 2.19 and The proofs of [23, Section 4, Theorems 7 and 8] work literally in our case for Theorems 2.19 and 2.20 respectively. As in the proof of Theorems 4.1 and 4.2 one should replace the reference to [23, Statements 1 and 2, p. 139] by references to Theorem 1.2 and to Corollaries 2.3 and 2.7.) 5. Torsion and ramification in solvable extensions 5.1. Let K be an arbitrary field and O K a discrete valuation ring whose field of fractions coincides with K. We write p for the maximal ideal of O and p for the characteristic of the residue field O/p. Let L/K be a finite Galois field extension with Galois group GalL/K) and O L the integral closure of O in L. The following assertion is well known see, e.g., [30, Chapter 5, Sections 7 10], [12, Chapter 1, Section 7]). i) O L is a principal ideal domain, the set of its maximal ideals is finite and consists of say) g maximal ideals q 1,..., q g such that t po = i=1 q i ) el/k) where el/k) is the weak) ramification index at p. The degree fl/k) = [O L /q i : O/p] of the field extension O L /q i )/O/p) equals the product f 0 p s where f 0 is the degree of the separable closure of O/p in O L /q i and s is a nonnegative integer that vanishes if and only if O L /q i is separable over O/p. The integers f 0 and s do not depend on i. The product el/k) f 0 p s g = el/k) f g = [L:K] = #GalL/K)). In particular, el/k) p s divides [L:K]. The field extension L/K is tamely ramified at p if and only if #Iq i )) is not divisible by p, i.e., el/k) is not divisible by p and O L /q i is separable over O/p. Here Iq i ) GalL/K) is the inertia subgroup attached to q i. ii) The Galois group GalL/K) acts transitively on the set {q i : 1 i g}. The corresponding inertia subgroups Iq i ) GalL/K) are conjugate subgroups of order el/k)p s in GalL/K). See [30, Chapter 5, Section 10, Theorem 24 and its proof].) iii) Let L 0 /K be a Galois subextension of L/K, i.e., L 0 /K is a Galois field extension and L 0 L. Then q 0 i = q i O L0 is a maximal ideal in O L0 that lies above p. The image of Iq i ) under the surjection GalL/K) GalL 0 /K) coincides with the inertia subgroup Iq 0 i ) GalL 0 /K) 670

13 Yu.G. Zarhin attached to q 0 i [12, Chapter 1, Section 7, Proposition 22 b)]; in particular, #Iq 0 i )) divides #Iq i )). On the other hand, #Iq 0 i )) divides #GalL 0 /K)) = [L 0 :K]. This implies that if [L 0 :K] and #Iq i )) are relatively prime then #Iq 0 i )) = 1, i.e., L 0 /K is unramified at p. Let Y be an abelian variety of positive dimension over K. Let n 3 be an integer that is not divisible by p. Assume that Y n Y K), i.e., all points of order n on X are defined over K. Let Y SpecO) be the Néron model of Y [1]; it is a smooth group scheme whose generic fiber coincides with Y. Since Y n Y K), the Raynaud criterion [5, Proposition 4.7] tells us that Y has semistable reduction at p, i.e., Y is a semiabelian group scheme. Let l be a prime different from p. For all positive integers j the field KY mj ) with m j = l j is a finite Galois extension. The following assertion was inspired by [8, Chapter XII, Section 2.0, p. 242]. Lemma 5.2. Let n 3 be an integer that is not divisible by p. Assume that Y n Y K). Let L = KY l ) and q be a maximal ideal in O L, which lies above p. Then: i) The inertia group Iq) is a finite commutative l-group. In particular, L/K is tamely ramified at p. ii) Let L 0 /K be a Galois subextension of L/K. If [L 0 :K] is not divisible by l then L 0 /K is unramified at p. Proof. The assertion ii) follows readily from i). So, let us prove i). For all positive integers j let us put L j = KY mj ) and O j = O Lj. We have L 1 = L, O 1 = O L. We have a tower of Galois extensions K L 1 L 2... L j... We write L for the union i=1 L i. For each j pick a maximal ideal q j) in O j in such a way that q 1) = q and q j+1) lies above q j). Such a choice is possible, because the projective limit of nonempty finite sets is nonempty.) Then the Galois group GalL /K) is the projective limit of finite groups GalL j /K). It is also clear that GalL j /K) = ρ mj,y GalK)) Aut Z/m j ZY mj ), GalL /K) = ρ l,y GalK)) Aut Zl T l Y )) Aut Ql V l Y )). Recall that the natural homomorphisms I q j+1)) I q j)) are surjective for all j. Let I be the projective limit of the corresponding inertia subgroups I q j)) ; clearly, I is a compact subgroup of GalL /K) and for each j the natural group homomorphism I I q j)) is surjective, because the projective limit of nonempty finite sets is also nonempty. Therefore one may view I as a certain compact subgroup of Aut Zl T l Y )) Aut Ql V l Y )). Since Y has semistable reduction at q, there exists a Q l -vector subspace W V l Y ) such that I acts trivially on W and V l Y )/W [5, Proposition 3.5]. It gives us an injective continuous homomorphism of topological groups I Hom Ql V l Y )/W, W), σ {v + W σv) v}, σ I, v V l Y ). Since I is compact, there is a continuous isomorphism between I and its image, which is a compact subgroup of Hom Ql V l Y )/W, W). Since the latter is a finite-dimensional Q l -vector space, all of its compact subgroups are commutative pro-l-groups that are isomorphic either to a direct sum of several copies of Z l or to zero). It follows that I is also a commutative pro-l-group. Since there is a surjective continuous homomorphism I I q j)), every I q j)) is a finite commutative l-group. This ends the proof, since q = q 1) and therefore Iq) = I q 1)). 671

14 Abelian varieties over fields of finite characteristic Proof of Theorem If K is finite then there is nothing to prove. So, let us assume that K is infinite. Let d 1 be the transcendence degree of K over F p. Let us pick a positive integer n 3 that is not divisible by p. Replacing K by KX n ) and X by X K KX n ), we may assume that X n XK). Let P be an infinite set of primes l p such that G l,x,k is l-solvable. By deleting finitely many primes from P, we may and will assume that the GalK)-module X l is semisimple for all l P. Since G l,x,k is the image of GalK) in Aut Fl X l ) = GL2g, F l ), the G l,x,k -module X l is semisimple for all l P. Let C be the field of complex numbers. Let us put g = dim X. Recall that X l is a 2g-dimensional F l -vector space. Let G be a finite l-solvable subgroup of Aut Fl X l ) such that the natural faithful representation of G in X l is completely reducible. Let us split the semisimple F l [G]-module X l into a direct sum X l = m i=1 W i of simple F l [G]-modules W i. If d i = dim Fl W i then 2g = m i=1 d i. By a theorem of Fong Swan [13, Section 16.3, Theorem 38], each W i lifts to characteristic zero; in particular, there is a group homomorphism ρ i : G GLd i, C) whose kernel lies in the kernel of G Aut Fl W i ) for all i). Clearly, the product-homomorphism d ρ = ρ i : G i=1 m GLd i, C) GL2g, C) i=1 is an injective group homomorphism and therefore G is isomorphic to a finite subgroup of GL2g, C). By a theorem of Jordan [2, Theorem 36.13], there is a positive integer N = N2g) that depends only on 2g and such that G contains a normal abelian subgroup of index dividing N. By deleting from P all prime divisors of N, we may and will assume that l does not divide N for each l P. Let us apply this observation to G = G l,x,k. We obtain that for all l P the group G l,x,k contains an abelian normal subgroup H l of index dividing N. For each l P let us consider the corresponding subfield of H l -invariants K l def = KX l )) H l KX l ). Clearly, K l /K is a Galois extension of degree dividing N while GalKX l )/K l ) coincides with commutative sub)group H l. Since K l KX l ) and [K l :K] divides N and therefore is not divisible by l, Lemma 5.2 tells us that the Galois extension K l /K is unramified with respect to every discrete valuation of K. Since chark) = p, its every residual characteristic is also p.) Let k be the finite) algebraic closure of F p in K and let S be an absolutely irreducible normal d-dimensional projective variety over k whose field of rational functions ks) coincides with K. Let V S be a smooth open dense subset such that the codimension of S \ V in S is, at least, 2. Let V l be the normalization of V in K l /K. Now the Zariski Nagata purity theorem tells us that the regular map V l V is an étale Galois cover; clearly, its degree equals [K l :K] and therefore divides N. Let π 1 V ) be the fundamental group of V that classifies étale covers of V [4], [8, Chapter XII, Section 1, pp ]). This group is a natural) topologically finitely generated topological) quotient of GalK) [8, Chapter XII, Section 1, p. 242] and the natural surjection GalK) GalK l /K) factors through π 1 V ), i.e., it is the composition of the canonical continuous surjection GalK) π 1 V ) and a certain continuous surjective homomorphism γ l : π 1 V ) GalK l /K) whose kernel is an open normal subgroup in π 1 V ) of index dividing N. 672

15 Yu.G. Zarhin Since π 1 V ) is topologically finitely generated, it contains only finitely many open normal subgroups of index dividing N because it admits only finitely many continuous homomorphisms to any finite group of order dividing N). If Γ is the intersection of all such subgroups then it is an open normal subgroup in π 1 V ) that lies in the kernel of every γ l for all l P; in particular, it has finite index. The preimage of Γ is an open normal subgroup of finite index in GalK) and the corresponding subfield of -invariants E def = K s is a finite Galois extension of K that contains K l for all l P. This implies that for all l P the compositum EKX l ) is abelian over E, because KX l ) is abelian over K l. But EKX l ) = EX E l) where X E = X K E is an abelian variety over E. Applying Theorem 2.11 to X E and E instead of X and K), we conclude that X E is an abelian variety of CM-type and isogenous over E = K to an abelian variety that is defined over a finite field. The same is true for X, since X E = X K E. 6. Concluding remarks Theorem 1.2 and Corollaries 2.3 iii) and 2.7 imply readily that the following results remain true for all prime characteristics p, including p = 2. Assertions Sections 1.3 and 4.4), Corollaries 1 6, Theorem 4.1, and Remark 1 of [20] remain true over any field E that is finitely generated over a finite field of arbitrary characteristic, including 2. [20, Corollary 7] remains true for any field F of arbitrary prime characteristic, including 2. [15, Theorem 1.1 ii)]. [28, Theorems 1.1, 1.4, 1.6, 1.9, 2.1]. 7. Corrigendum to [27] P. 317, Remark 1.3, second sentence: one has to assume additionally that the kernel of the morphism is W. P. 317, line -12: read HomY t, X t ) instead of HomY, X). P. 326, Proof of Theorem 3.4, second line: read 8g-dimensional instead of 4g-dimensional. Acknowledgements The final version of this paper was prepared during a stay at Max-Planck-Institut für Mathematik Bonn) in September 2013: the author is grateful to the MPI for the hospitality and support. The author is grateful to Alexey Parshin, Chad Schoen and Doug Ulmer for their interest in this paper. Special thanks go to the referees, whose comments helped to improve the exposition. This work was partially supported by a grant from the Simons Foundation # to Yuri Zarkhin). References [1] Bosch S., Lütkebohmert W., Raynaud M., Néron Models, Ergeb. Math. Grenzgeb., 21, Springer, Berlin,

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