Tate module

In mathematics, a Tate module of an abelian group, named for John Tate, is a module constructed from an abelian group A. Often, this construction is made in the following situation: G is a commutative group scheme over a field K, K^s is the separable closure of K, and A = G(K^s) (the K^s-valued points of G). In this case, the Tate module of A is equipped with an action of the absolute Galois group of K, and it is referred to as the Tate module of G.

Not to be confused with Hodge–Tate module.

Definition

Given an abelian group A and a prime number p, the p-adic Tate module of A is

${\displaystyle T_{p}(A)={\underset {\longleftarrow }{\lim }}A[p^{n}]}$

where A[pⁿ] is the pⁿ torsion of A (i.e. the kernel of the multiplication-by-pⁿ map), and the inverse limit is over positive integers n with transition morphisms given by the multiplication-by-p map A[pⁿ⁺¹] → A[pⁿ]. Thus, the Tate module encodes all the p-power torsion of A. It is equipped with the structure of a Z_p-module via

${\displaystyle z(a_{n})_{n}=((z{\text{ mod }}p^{n})a_{n})_{n}.}$

Examples

The Tate module

When the abelian group A is the group of roots of unity in a separable closure K^s of K, the p-adic Tate module of A is sometimes referred to as the Tate module (where the choice of p and K are tacitly understood). It is a free rank one module over Z_p with a linear action of the absolute Galois group G_K of K. Thus, it is a Galois representation also referred to as the p-adic cyclotomic character of K. It can also be considered as the Tate module of the multiplicative group scheme G_m,K over K.

The Tate module of an abelian variety

Given an abelian variety G over a field K, the K^s-valued points of G are an abelian group. The p-adic Tate module T_p(G) of G is a Galois representation (of the absolute Galois group, G_K, of K).

Classical results on abelian varieties show that if K has characteristic zero, or characteristic ℓ where the prime number p ≠ ℓ, then T_p(G) is a free module over Z_p of rank 2d, where d is the dimension of G.[1] In the other case, it is still free, but the rank may take any value from 0 to d (see for example Hasse–Witt matrix).

In the case where p is not equal to the characteristic of K, the p-adic Tate module of G is the dual of the étale cohomology ${\displaystyle H_{\text{et}}^{1}(G\times _{K}K^{s},\mathbf {Z} _{p})}$.

A special case of the Tate conjecture can be phrased in terms of Tate modules.[2] Suppose K is finitely generated over its prime field (e.g. a finite field, an algebraic number field, a global function field), of characteristic different from p, and A and B are two abelian varieties over K. The Tate conjecture then predicts that

${\displaystyle \mathrm {Hom} _{K}(A,B)\otimes \mathbf {Z} _{p}\cong \mathrm {Hom} _{G_{K}}(T_{p}(A),T_{p}(B))}$

where Hom_K(A, B) is the group of morphisms of abelian varieties from A to B, and the right-hand side is the group of G_K-linear maps from T_p(A) to T_p(B). The case where K is a finite field was proved by Tate himself in the 1960s.[3] Gerd Faltings proved the case where K is a number field in his celebrated "Mordell paper".[4]

In the case of a Jacobian over a curve C over a finite field k of characteristic prime to p, the Tate module can be identified with the Galois group of the composite extension

${\displaystyle k(C)\subset {\hat {k}}(C)\subset A^{(p)}\ }$

where ${\displaystyle {\hat {k}}}$ is an extension of k containing all p-power roots of unity and A^(p) is the maximal unramified abelian p-extension of ${\displaystyle {\hat {k}}(C)}$.[5]

Tate module of a number field

The description of the Tate module for the function field of a curve over a finite field suggests a definition for a Tate module of an algebraic number field, the other class of global field, introduced by Kenkichi Iwasawa. For a number field K we let K_m denote the extension by p^m-power roots of unity, ${\displaystyle {\hat {K}}}$ the union of the K_m and A^(p) the maximal unramified abelian p-extension of ${\displaystyle {\hat {K}}}$. Let

${\displaystyle T_{p}(K)=\mathrm {Gal} (A^{(p)}/{\hat {K}})\ .}$

Then T_p(K) is a pro-p-group and so a Z_p-module. Using class field theory one can describe T_p(K) as isomorphic to the inverse limit of the class groups C_m of the K_m under norm.[5]

Iwasawa exhibited T_p(K) as a module over the completion Z_p[[T]] and this implies a formula for the exponent of p in the order of the class groups C_m of the form

${\displaystyle \lambda m+\mu p^{m}+\kappa \ .}$

The Ferrero–Washington theorem states that μ is zero.[6]

See also

• Tate conjecture
• Tate twist

Notes

1. Murty 2000, Proposition 13.4
2. Murty 2000, §13.8
3. Tate 1966
4. Faltings 1983
5. Manin & Panchishkin 2007, p. 245 harvnb error: no target: CITEREFManinPanchishkin2007 (help)
6. Manin & Panchishkin 2007, p. 246 harvnb error: no target: CITEREFManinPanchishkin2007 (help)

References

• Faltings, Gerd (1983), "Endlichkeitssätze für abelsche Varietäten über Zahlkörpern", Inventiones Mathematicae, 73 (3): 349–366, doi:10.1007/BF01388432
• "Tate module", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
• Murty, V. Kumar (2000), Introduction to abelian varieties, CRM Monograph Series, 3, American Mathematical Society, ISBN 978-0-8218-1179-5
• Section 13 of Rohrlich, David (1994), "Elliptic curves and the Weil–Deligne group", in Kisilevsky, Hershey; Murty, M. Ram (eds.), Elliptic curves and related topics, CRM Proceedings and Lecture Notes, 4, American Mathematical Society, ISBN 978-0-8218-6994-9
• Tate, John (1966), "Endomorphisms of abelian varieties over finite fields", Inventiones Mathematicae, 2: 134–144, doi:10.1007/bf01404549, MR 0206004