Tate cohomology group

Definition

If G is a finite group and A a G-module, then there is a natural map N from H₀(G,A) to H⁰(G,A) taking a representative a to Σga (the sum over all G-conjugates of a). The Tate cohomology groups H ^ n ( G , A ) are defined by

H ^ n ( G , A ) = H n ( G , A ) for n≥ 1,

H ^ 0 ( G , A ) = coker ⁡ N = quotient of H⁰(G,A) by norms of elements of A,

H ^ − 1 ( G , A ) = ker ⁡ N = quotient of norm 0 elements of A by principal elements of A,

H ^ n ( G , A ) = H − ( n + 1 ) ( G , A ) for n≤ −2.

Properties

If

0 ⟶ A ⟶ B ⟶ C ⟶ 0

is a short exact sequence of G-modules, then we get the usual long exact sequence of Tate cohomology groups:

⋯ ⟶ H ^ n ( G , A ) ⟶ H ^ n ( G , B ) ⟶ H ^ n ( G , C ) ⟶ H ^ n + 1 ( G , A ) ⟶ H ^ n + 1 ( G , B ) ⋯

If A is an induced G module then all Tate cohomology groups of A vanish.

The zeroth Tate cohomology group of A is

(Fixed points of G on A)/(Obvious fixed points of G acting on A)

where by the "obvious" fixed point we mean those of the form Σ g(a). In other words, the zeroth cohomology group in some sense describes the non-obvious fixed points of G acting on A.

The Tate cohomology groups are characterized by the three properties above.

Tate's theorem

Tate's theorem (Tate 1952) gives conditions for multiplication by a cohomology class to be an isomorphism between cohomology groups. There are several slightly different versions of it; a version that is particularly convenient for class field theory is as follows:

Suppose that A is a module over a finite group G and a is an element of H²(G,A), such that for every subgroup E of G

H¹(E,A) is trivial, and

H²(E,A) is generated by Res(a) which has order E.

Then cup product with a is an isomorphism

H ^ n ( G , Z ) ⟶ H ^ n + 2 ( G , A )

for all n; in other words the graded Tate cohomology of A is isomorphic to the Tate cohomology with integral coefficients, with the degree shifted by 2.

Tate-Farrell cohomology

Farrell extended Tate cohomology groups to the case of all groups G of finite virtual cohomological dimension. In Farrell's theory, the groups H ^ n ( G , A ) are isomorphic to the usual cohomology groups whenever n is greater than the virtual cohomological dimension of the group G. Finite groups have virtual cohomological dimension 0, and in this case Farrell's cohomology groups are the same as those of Tate.