Arithmetic of certain integrable systems. University of Chicago & Vietnam Institute for Advanced Study in Mathematics

Transcription

1 Arithmetic of certain integrable systems Ngô Bao Châu University of Chicago & Vietnam Institute for Advanced Study in Mathematics

2 System of congruence equations Let us consider a system of congruence equations P 1 (x 1,..., x n ) = 0 P m (x 1,..., x n ) = 0

3 System of congruence equations Let us consider a system of congruence equations P 1 (x 1,..., x n ) = 0 P m (x 1,..., x n ) = 0 where P 1,..., P m F p [x 1,..., x n ] are polynomial with coefficients in F p = Z/pZ.

4 System of congruence equations Let us consider a system of congruence equations P 1 (x 1,..., x n ) = 0 P m (x 1,..., x n ) = 0 where P 1,..., P m F p [x 1,..., x n ] are polynomial with coefficients in F p = Z/pZ. We are interested in the number of solutions of this system with in F p, and more generally in F p r where F p r is the finite extension of degree r of F p.

5 Valued points of algebraic variety If we denote X = SpecF p [x 1,..., x n ]/(P 1,..., P m ), the algebraic variety defined by the system of equations P 1 = 0,..., P m = 0, then X (F r p) is the set of solutions with values in F p r.

6 Valued points of algebraic variety If we denote X = SpecF p [x 1,..., x n ]/(P 1,..., P m ), the algebraic variety defined by the system of equations P 1 = 0,..., P m = 0, then X (F r p) is the set of solutions with values in F p r. Let X ( F p ) = r N X (F p r ) be the set of points with values in the algebraic closure F p of F p

7 Valued points of algebraic variety If we denote X = SpecF p [x 1,..., x n ]/(P 1,..., P m ), the algebraic variety defined by the system of equations P 1 = 0,..., P m = 0, then X (F r p) is the set of solutions with values in F p r. Let X ( F p ) = r N X (F pr ) be the set of points with values in the algebraic closure F p of F p The Galois group Gal( F p /F p ) acts on X ( F p ). It is generated by the Frobenius element σ(x) = x p, and Fix(σ r, X ( F p )) = X (F p r ).

8 Grothendieck-Lefschetz formula For a prime number l p, Grothendieck defined the groups of l-adic cohomology of X H i (X ) = H i (X Fp F p, Q l ) and H i c = H i c(x Fp F p, Q l ) for every algebraic variety X over F p

9 Grothendieck-Lefschetz formula For a prime number l p, Grothendieck defined the groups of l-adic cohomology of X H i (X ) = H i (X Fp F p, Q l ) and H i c = H i c(x Fp F p, Q l ) for every algebraic variety X over F p and proved the Lefschetz fixed points formula #Fix(σ r p, X ( F p )) = 2 dim(x ) i=0 ( 1) i tr(σ r p, H i c(x )).

10 Grothendieck-Lefschetz formula For a prime number l p, Grothendieck defined the groups of l-adic cohomology of X H i (X ) = H i (X Fp F p, Q l ) and H i c = H i c(x Fp F p, Q l ) for every algebraic variety X over F p and proved the Lefschetz fixed points formula #Fix(σ r p, X ( F p )) = 2 dim(x ) i=0 ( 1) i tr(σ r p, H i c(x )). Deligne proved that for every field isomorphism ι : Q l C, the inequality ι(α) p i/2 for all eigenvalues α of σ acting on H i c(x ).

11 Equality of numbers of points We will be concerned with proving equality of type #X (F p r ) = #X (F p r ) for different algebraic varieties.

12 Equality of numbers of points We will be concerned with proving equality of type #X (F p r ) = #X (F p r ) for different algebraic varieties. We would like to develop a principle of analytic continuation of equalities : Let f : X Y and f : X Y be morphisms of algebraic varieties. If the equality #X y (F q r ) = #X y (F q r ) holds for every point y in a dense open subset U of Y, then it holds for every y Y.

13 Equality of numbers of points We will be concerned with proving equality of type #X (F p r ) = #X (F p r ) for different algebraic varieties. We would like to develop a principle of analytic continuation of equalities : Let f : X Y and f : X Y be morphisms of algebraic varieties. If the equality #X y (F q r ) = #X y (F q r ) holds for every point y in a dense open subset U of Y, then it holds for every y Y. This can t be true in general. The question is to find geometric assumptions on f and f that guarantee this principle.

14 Equality of numbers of points We will be concerned with proving equality of type #X (F p r ) = #X (F p r ) for different algebraic varieties. We would like to develop a principle of analytic continuation of equalities : Let f : X Y and f : X Y be morphisms of algebraic varieties. If the equality #X y (F q r ) = #X y (F q r ) holds for every point y in a dense open subset U of Y, then it holds for every y Y. This can t be true in general. The question is to find geometric assumptions on f and f that guarantee this principle. The complex of l-adic sheaves f! Q l interpolates all cohomology group with compact support H i c(x y ) H i (f! Q l ) y = H i c(x y ) for all geometric points y Y. Geometric assumption on f give constraint on the copmlex f! Q l.

15 The case of proper and smooth morphisms Let f : X Y and f : X Y be proper and smooth morphisms. Assume that there exists an dense open subset U of Y, such that for all y U(F q r ), #X y (F q r ) = #X y (F q r ).

16 The case of proper and smooth morphisms Let f : X Y and f : X Y be proper and smooth morphisms. Assume that there exists an dense open subset U of Y, such that for all y U(F q r ), #X y (F q r ) = #X y (F q r ). If f : X Y and f : X Y are proper and smooth morphisms then H i (f! Q l ) and H i (f! Q l) are l-adic local systems for every i Z.

17 The case of proper and smooth morphisms Let f : X Y and f : X Y be proper and smooth morphisms. Assume that there exists an dense open subset U of Y, such that for all y U(F q r ), #X y (F q r ) = #X y (F q r ). If f : X Y and f : X Y are proper and smooth morphisms then H i (f! Q l ) and H i (f! Q l) are l-adic local systems for every i Z. Deligne s theorem implies that tr(σ y, H i c(x y )) = tr(σ y, H i c(x y )).

18 The case of proper and smooth morphisms Let f : X Y and f : X Y be proper and smooth morphisms. Assume that there exists an dense open subset U of Y, such that for all y U(F q r ), #X y (F q r ) = #X y (F q r ). If f : X Y and f : X Y are proper and smooth morphisms then H i (f! Q l ) and H i (f! Q l) are l-adic local systems for every i Z. Deligne s theorem implies that tr(σ y, H i c(x y )) = tr(σ y, H i c(x y )). The Chebotarev density theorem implies that the l-adic local systems H i (f! Q l ) and H i (f! Q l) are isomorphic.

19 The case of proper and smooth morphisms Let f : X Y and f : X Y be proper and smooth morphisms. Assume that there exists an dense open subset U of Y, such that for all y U(F q r ), #X y (F q r ) = #X y (F q r ). If f : X Y and f : X Y are proper and smooth morphisms then H i (f! Q l ) and H i (f! Q l) are l-adic local systems for every i Z. Deligne s theorem implies that tr(σ y, H i c(x y )) = tr(σ y, H i c(x y )). The Chebotarev density theorem implies that the l-adic local systems H i (f! Q l ) and H i (f! Q l) are isomorphic. A local system is determined by its restriction to any dense open subset.

20 Singularities To obtain interesting cases, one has to drop the smoothness assumption.

21 Singularities To obtain interesting cases, one has to drop the smoothness assumption. Goresky-MacPherson s theory of perverse sheaves is very efficient in dealing with singularities of algebraic maps.

22 Singularities To obtain interesting cases, one has to drop the smoothness assumption. Goresky-MacPherson s theory of perverse sheaves is very efficient in dealing with singularities of algebraic maps. For every algebraic variety Y, the category P(Y ) of perverse sheaves of Y is an abelian categories. For every morphism f : X Y, one can define perverse cohomology p H i (f! Q l ) P(Y ) in similar way as usual cohomology H i (f! Q l ) are usual l-adic sheaves.

23 Purity and semi-simplicity Let f : X Y be a proper morphism where X is a smooth variety. Then according to Deligne, f! Q l is a pure complex of sheaves.

24 Purity and semi-simplicity Let f : X Y be a proper morphism where X is a smooth variety. Then according to Deligne, f! Q l is a pure complex of sheaves. As important consequence of Deligne s purity theorem, Beilinson, Bernstein, Deligne and Gabber proved that after base change to Y F p, p H i (f! Q l ) is a direct sum of simple perverse sheaves.

25 Purity and semi-simplicity Let f : X Y be a proper morphism where X is a smooth variety. Then according to Deligne, f! Q l is a pure complex of sheaves. As important consequence of Deligne s purity theorem, Beilinson, Bernstein, Deligne and Gabber proved that after base change to Y F p, p H i (f! Q l ) is a direct sum of simple perverse sheaves. There exists over Y F p a decomposition in direct sum f! Q l = α A K α [n α ] where K α are simple perverse sheaves and n α Z.

26 Simple perverse sheaves Let i : Z Y Fp F p be the immersion of an irreducible closed irreducible subscheme. Let j : U Z be the immersion of a nonempty open subscheme. Let L be an irreducible local system on U, then K = i j! L[dim Z] is a simple perverse sheaf on Y Fp F p.

27 Simple perverse sheaves Let i : Z Y Fp F p be the immersion of an irreducible closed irreducible subscheme. Let j : U Z be the immersion of a nonempty open subscheme. Let L be an irreducible local system on U, then K = i j! L[dim Z] is a simple perverse sheaf on Y Fp F p. According to Goresky and MacPherson, every simple perverse sheaf is of this form.

28 Simple perverse sheaves Let i : Z Y Fp F p be the immersion of an irreducible closed irreducible subscheme. Let j : U Z be the immersion of a nonempty open subscheme. Let L be an irreducible local system on U, then K = i j! L[dim Z] is a simple perverse sheaf on Y Fp F p. According to Goresky and MacPherson, every simple perverse sheaf is of this form. The definition of the intermediate extension functot j! is complicated. For us, what really matters is that the perverse sheaf is completely determined by the local system L, more generally, it is determined by the restriction of L to any nonempty open subscheme of U.

29 Support If K is a simple perverse sheaf on Y F p, it is of the form K = i j! L[dim Z]. In particular, supp(k) := Z is completely determined.

30 Support If K is a simple perverse sheaf on Y F p, it is of the form K = i j! L[dim Z]. In particular, supp(k) := Z is completely determined. Let f : X Y be a proper morphism where X is a smooth variety. then, f! Q l can be decomposed into a direct sum f! Q l = α A K α [n α ] of simple perverse sheaves. The finite set supp(f ) = {Z α Z α = supp(k α )} is well determined. This is an important topological invariant of f.

31 Only full support Let f : X Y and f : X Y be proper morphisms with X, X smooth varieties. If supp(f ) = supp(f ) = {Y }, then the analytic continuation principle applies as f! Q l and f! Q l are determined by their restrictions to any nonempty open subset.

32 Only full support Let f : X Y and f : X Y be proper morphisms with X, X smooth varieties. If supp(f ) = supp(f ) = {Y }, then the analytic continuation principle applies as f! Q l and f! Q l are determined by their restrictions to any nonempty open subset. This is true if f and f are proper and smooth.

33 Only full support Let f : X Y and f : X Y be proper morphisms with X, X smooth varieties. If supp(f ) = supp(f ) = {Y }, then the analytic continuation principle applies as f! Q l and f! Q l are determined by their restrictions to any nonempty open subset. This is true if f and f are proper and smooth. Are there more interesting cases?

34 Small map f : X Y is small in the sense of Goresky and MacPherson if dim(x Y X X ) < dim(x ).

35 Small map f : X Y is small in the sense of Goresky and MacPherson if dim(x Y X X ) < dim(x ). Goresky and MacPherson proved that if f : X Y is a small proper map and if X is smooth, then f! Q l is a perverse sheaf which is the intermediate extension of its restriction to any dense open subset.

36 Small map f : X Y is small in the sense of Goresky and MacPherson if dim(x Y X X ) < dim(x ). Goresky and MacPherson proved that if f : X Y is a small proper map and if X is smooth, then f! Q l is a perverse sheaf which is the intermediate extension of its restriction to any dense open subset. In particular supp(f ) = {Y }

37 Small map f : X Y is small in the sense of Goresky and MacPherson if dim(x Y X X ) < dim(x ). Goresky and MacPherson proved that if f : X Y is a small proper map and if X is smooth, then f! Q l is a perverse sheaf which is the intermediate extension of its restriction to any dense open subset. In particular supp(f ) = {Y } Argument: play the Poincaré duality against the cohomological amplitude.

38 Relative curve Let f : X Y be a relative curve such that X is smooth, f is proper, for generic y Y, X y is smooth and for every y Y, X y is irreducible.

39 Relative curve Let f : X Y be a relative curve such that X is smooth, f is proper, for generic y Y, X y is smooth and for every y Y, X y is irreducible. Then according to Goresky and MacPherson, supp(f ) = {Y }.

40 Relative curve Let f : X Y be a relative curve such that X is smooth, f is proper, for generic y Y, X y is smooth and for every y Y, X y is irreducible. Then according to Goresky and MacPherson, supp(f ) = {Y }. Argument: play the Poincaré duality against the cohomological amplitude.

41 Poincaré duality versus cohomological amplitude Assume for simplicity dim(x ) = 2 and dim(y ) = 1.

42 Poincaré duality versus cohomological amplitude Assume for simplicity dim(x ) = 2 and dim(y ) = 1. The cohomological amplitude of a relative curve: H i (f! Q l [2]) = 0 for i / { 2, 1, 0}

43 Poincaré duality versus cohomological amplitude Assume for simplicity dim(x ) = 2 and dim(y ) = 1. The cohomological amplitude of a relative curve: H i (f! Q l [2]) = 0 for i / { 2, 1, 0} Assume there exists a simple perverse sheaf K α such that K α [n α ] is a direct factor of f! Q l [ 2] and dim(z α ) = 0 where Z α = supp(k α ).

44 Poincaré duality versus cohomological amplitude Assume for simplicity dim(x ) = 2 and dim(y ) = 1. The cohomological amplitude of a relative curve: H i (f! Q l [2]) = 0 for i / { 2, 1, 0} Assume there exists a simple perverse sheaf K α such that K α [n α ] is a direct factor of f! Q l [ 2] and dim(z α ) = 0 where Z α = supp(k α ). H 0 (K α ) 0, the cohomological amplitude implies that n α 0.

45 Poincaré duality versus cohomological amplitude Assume for simplicity dim(x ) = 2 and dim(y ) = 1. The cohomological amplitude of a relative curve: H i (f! Q l [2]) = 0 for i / { 2, 1, 0} Assume there exists a simple perverse sheaf K α such that K α [n α ] is a direct factor of f! Q l [ 2] and dim(z α ) = 0 where Z α = supp(k α ). H 0 (K α ) 0, the cohomological amplitude implies that n α 0. By Poincaré duality K α [ n α ] is also a direct factor of f! Q l [2] where supp(k α ) = supp(k α ). It follows that n α 0.

46 Poincaré duality versus cohomological amplitude Assume for simplicity dim(x ) = 2 and dim(y ) = 1. The cohomological amplitude of a relative curve: H i (f! Q l [2]) = 0 for i / { 2, 1, 0} Assume there exists a simple perverse sheaf K α such that K α [n α ] is a direct factor of f! Q l [ 2] and dim(z α ) = 0 where Z α = supp(k α ). H 0 (K α ) 0, the cohomological amplitude implies that n α 0. By Poincaré duality K α [ n α ] is also a direct factor of f! Q l [2] where supp(k α ) = supp(k α ). It follows that n α 0. It follows that n α = 0. But then H 0 (K α ) is a direct factor of H 2 (f! Q l ) = Q l ( 1). This is not possible.

47 Goresky-MacPherson s inequality Let f : X Y be a proper morphism with fiber of dimension d. Assume X smooth. Let Z supp(f ) be the support of a perverse direct factor of f! Q l.

48 Goresky-MacPherson s inequality Let f : X Y be a proper morphism with fiber of dimension d. Assume X smooth. Let Z supp(f ) be the support of a perverse direct factor of f! Q l. Then codim(z) d.

49 Goresky-MacPherson s inequality Let f : X Y be a proper morphism with fiber of dimension d. Assume X smooth. Let Z supp(f ) be the support of a perverse direct factor of f! Q l. Then codim(z) d. Moreover, if the geometric fibers of f are irreducible, then codim(z) < d.

50 Goresky-MacPherson s inequality Let f : X Y be a proper morphism with fiber of dimension d. Assume X smooth. Let Z supp(f ) be the support of a perverse direct factor of f! Q l. Then codim(z) d. Moreover, if the geometric fibers of f are irreducible, then codim(z) < d. For abelian fibration, Goresky-MacPherson s inequality can be used to establish the full support theorem.

51 Weak abelian fibration f : M S is a proper morphism, g : P S is a smooth commutative group scheme, both of relative dimension d,

52 Weak abelian fibration f : M S is a proper morphism, g : P S is a smooth commutative group scheme, both of relative dimension d, P acts on M relatively over S.

53 Weak abelian fibration f : M S is a proper morphism, g : P S is a smooth commutative group scheme, both of relative dimension d, P acts on M relatively over S. We assume that the action has affine stabilizers: for every geometric point s S, for every m M s, the stabilizer P m is affine.

54 Weak abelian fibration f : M S is a proper morphism, g : P S is a smooth commutative group scheme, both of relative dimension d, P acts on M relatively over S. We assume that the action has affine stabilizers: for every geometric point s S, for every m M s, the stabilizer P m is affine. We assume that the Tate modules of P is polarizable.

55 Tate module in family Assume P has connected fibers, for every geometric point s S, there exists a canonical exact sequence 0 R s P s A s 0 where A s is an abelian variety and R s is a connected affine group. This induces an exact sequence of Tate modules 0 T Ql (R s ) T Ql (P s ) T Ql (A s ) 0.

56 Tate module in family Assume P has connected fibers, for every geometric point s S, there exists a canonical exact sequence 0 R s P s A s 0 where A s is an abelian variety and R s is a connected affine group. This induces an exact sequence of Tate modules 0 T Ql (R s ) T Ql (P s ) T Ql (A s ) 0. The Tate modules can be interpolated into a single l-adic sheaf H 1 (P/S) = H 2d 1 (g! Q l ) with fiber H 1 (P/S) s = T Ql (P s ). Polarization of the Tate module of P is an alternating form on H 1 (P/S) vanishing on T Ql (R s ) and induces a perfect pairing on T Ql (A s ).

57 δ-regularity For every geometric point s S, we define δ(s) = dim(r s ) the dimension of the affine part of P s.

58 δ-regularity For every geometric point s S, we define δ(s) = dim(r s ) the dimension of the affine part of P s. For every δ N, is locally closed. S δ = {s S δ(s) = δ}

59 δ-regularity For every geometric point s S, we define δ(s) = dim(r s ) the dimension of the affine part of P s. For every δ N, is locally closed. S δ = {s S δ(s) = δ} P S is said to be δ-regular if codim(s δ ) δ for every δ N.

60 δ-regularity For every geometric point s S, we define δ(s) = dim(r s ) the dimension of the affine part of P s. For every δ N, is locally closed. S δ = {s S δ(s) = δ} P S is said to be δ-regular if codim(s δ ) δ for every δ N. In particular, for δ = 1, the δ-regularity means P is generically an abelian variety.

61 δ-regularity For every geometric point s S, we define δ(s) = dim(r s ) the dimension of the affine part of P s. For every δ N, is locally closed. S δ = {s S δ(s) = δ} P S is said to be δ-regular if codim(s δ ) δ for every δ N. In particular, for δ = 1, the δ-regularity means P is generically an abelian variety. One can prove δ-regularity for all Hamiltonian completely integrable system.

62 δ-regularity For every geometric point s S, we define δ(s) = dim(r s ) the dimension of the affine part of P s. For every δ N, is locally closed. S δ = {s S δ(s) = δ} P S is said to be δ-regular if codim(s δ ) δ for every δ N. In particular, for δ = 1, the δ-regularity means P is generically an abelian variety. One can prove δ-regularity for all Hamiltonian completely integrable system. δ-regularity is harder to prove in characteristic p.

63 Theorem of support for abelian fibration Theorem: Let (f : M S, g : P S) be a δ-regular abelian fibration. Assume that M is smooth, the fibers of f : M S are irreducible. Then supp(f ) = {S}.

64 Theorem of support for abelian fibration Theorem: Let (f : M S, g : P S) be a δ-regular abelian fibration. Assume that M is smooth, the fibers of f : M S are irreducible. Then supp(f ) = {S}. Corollary: Let (P, M, S) and (P, M, S) be δ-regular abelian fibrations as above (in particular, M s and M s are irreducible). If the generic fibers of P and P are isogenous abelian varieties, then for every s S(F q ), #M s (F q ) = #M s(f q ).

65 Theorem of support for abelian fibration Theorem: Let (f : M S, g : P S) be a δ-regular abelian fibration. Assume that M is smooth, the fibers of f : M S are irreducible. Then supp(f ) = {S}. Corollary: Let (P, M, S) and (P, M, S) be δ-regular abelian fibrations as above (in particular, M s and M s are irreducible). If the generic fibers of P and P are isogenous abelian varieties, then for every s S(F q ), #M s (F q ) = #M s(f q ). Remark: In practice, one has to drop the condition M s irreducible and P s connected. In these cases, the formulations of the support theorem and the numerical equality are more complicated.

66 Theorem of support for abelian fibration Theorem: Let (f : M S, g : P S) be a δ-regular abelian fibration. Assume that M is smooth, the fibers of f : M S are irreducible. Then supp(f ) = {S}. Corollary: Let (P, M, S) and (P, M, S) be δ-regular abelian fibrations as above (in particular, M s and M s are irreducible). If the generic fibers of P and P are isogenous abelian varieties, then for every s S(F q ), #M s (F q ) = #M s(f q ). Remark: In practice, one has to drop the condition M s irreducible and P s connected. In these cases, the formulations of the support theorem and the numerical equality are more complicated. This theorem is the key geometric ingredient in the proof of Langlands fundamental lemma.

67 Upper bound on codimension For every closed irreducible subscheme Z of S, we set δ Z = min s S δ(s).

68 Upper bound on codimension For every closed irreducible subscheme Z of S, we set δ Z = min s S δ(s). We prove that if Z supp(f ), then codim(z) δ Z and if the geometric fibers of f : M S are irreducible then codim(z) < δ Z unless Z = S.

69 Upper bound on codimension For every closed irreducible subscheme Z of S, we set δ Z = min s S δ(s). We prove that if Z supp(f ), then codim(z) δ Z and if the geometric fibers of f : M S are irreducible then codim(z) < δ Z unless Z = S. By the δ-regularity, we have the inequality codim(z) δ Z. The only possibility is Z = S.

70 Topological explanation Let s Z such that δ(s) = δ Z.

71 Topological explanation Let s Z such that δ(s) = δ Z. Assume there is a splitting A s P S of the Chevalley exact sequence 0 R s P s A s 0

72 Topological explanation Let s Z such that δ(s) = δ Z. Assume there is a splitting A s P S of the Chevalley exact sequence 0 R s P s A s 0 Assume there exists an étale neighborhood S of s, an abelian scheme A S of special fiber A s, and a homormorphism A P extending the splitting A s P s.

73 Topological explanation Let s Z such that δ(s) = δ Z. Assume there is a splitting A s P S of the Chevalley exact sequence 0 R s P s A s 0 Assume there exists an étale neighborhood S of s, an abelian scheme A S of special fiber A s, and a homormorphism A P extending the splitting A s P s. Then over S, A acts almost freely on M and on can factorize M S as M [M /A ] S where the morphism M [M /A ] is proper and smooth, and the morphism [M /A ] S is of relative dimension δ s. Thus our inequality can be reduced to the Goresky-MacPherson inequality.

74 Implement the argument The assumptions are not satisfied in general. The generic fiber of P is usually an irreducible abelian variety and does not admit a factor of smaller dimension i.e the lifting A P can t exist.

75 Implement the argument The assumptions are not satisfied in general. The generic fiber of P is usually an irreducible abelian variety and does not admit a factor of smaller dimension i.e the lifting A P can t exist. To overcome this difficulty, one need to reformulate the above argument in terms of homological algebra instead of topology.