Tate conjecture


In number theory and algebraic geometry, the Tate conjecture is a 1963 conjecture of John Tate that would describe the algebraic cycles on a variety in terms of a more computable invariant, the Galois representation on étale cohomology. The conjecture is a central problem in the theory of algebraic cycles. It can be considered an arithmetic analog of the Hodge conjecture.

Statement of the conjecture

Let V be a smooth projective variety over a field k which is finitely generated over its prime field. Let ks be a separable closure of k, and let G be the absolute Galois group Gal of k. Fix a prime number ℓ which is invertible in k. Consider the ℓ-adic cohomology groups of the base extension of V to ks; these groups are representations of G. For any i ≥ 0, a codimension-i subvariety of V determines an element of the cohomology group
which is fixed by G. Here Qℓ denotes the ith Tate twist, which means that this representation of the Galois group G is tensored with the ith power of the cyclotomic character.
The Tate conjecture states that the subspace WG of W fixed by the Galois group G is spanned, as a Qℓ-vector space, by the classes of codimension-i subvarieties of V. An algebraic cycle means a finite linear combination of subvarieties; so an equivalent statement is that every element of WG is the class of an algebraic cycle on V with Qℓ coefficients.

Known cases

The Tate conjecture for divisors is a major open problem. For example, let f : X → C be a morphism from a smooth projective surface onto a smooth projective curve over a finite field. Suppose that the generic fiber F of f, which is a curve over the function field k, is smooth over k. Then the Tate conjecture for divisors on X is equivalent to the Birch-Swinnerton-Dyer conjecture for the Jacobian variety of F. By contrast, the Hodge conjecture for divisors on any smooth complex projective variety is known.
Probably the most important known case is that the Tate conjecture is true for divisors on abelian varieties. This is a theorem of Tate for abelian varieties over finite fields, and of Faltings for abelian varieties over number fields, part of Faltings's solution of the Mordell conjecture. Zarhin extended these results to any finitely generated base field. The Tate conjecture for divisors on abelian varieties implies the Tate conjecture for divisors on any product of curves C1 ×... × Cn.
The Tate conjecture for divisors on abelian varieties is equivalent to a powerful statement about homomorphisms between abelian varieties. Namely, for any abelian varieties A and B over a finitely generated field k, the natural map
is an isomorphism. In particular, an abelian variety A is determined up to isogeny by the Galois representation on its Tate module H1.
The Tate conjecture also holds for K3 surfaces over finitely generated fields of characteristic not 2. In characteristic zero, the Tate conjecture for K3 surfaces was proved by André and Tankeev. For K3 surfaces over finite fields of characteristic not 2, the Tate conjecture was proved by Nygaard, Ogus, Charles, Madapusi Pera, and Maulik.
surveys known cases of the Tate conjecture.

Related conjectures

Let X be a smooth projective variety over a finitely generated field k. The semisimplicity conjecture predicts that the representation of the Galois group G = Gal on the ℓ-adic cohomology of X is semisimple. For k of characteristic 0, showed that the Tate conjecture implies the semisimplicity of
For k finite of order q, Tate showed that the Tate conjecture plus the semisimplicity conjecture would imply the strong Tate conjecture, namely that the order of the pole of the zeta function Z at t = q−j is equal to the rank of the group of algebraic cycles of codimension j modulo numerical equivalence.
Like the Hodge conjecture, the Tate conjecture would imply most of Grothendieck's standard conjectures on algebraic cycles. Namely, it would imply the Lefschetz standard conjecture ; that the Künneth components of the diagonal are algebraic; and that numerical equivalence and homological equivalence of algebraic cycles are the same.