Give an example of a $G$-module $M$, such that $\hat{H}^{*}(G,M)=0$, but $M$ is not cohomologically trivial. Here $\hat{H}^{*}(G,-)$ means Tate Cohomology.
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This hint is from Serre's Local fields:
Take $G$ to be the cyclic group of order 6 and let $A = \mathbb{Z}/3\mathbb{Z}$. Let $G$ operate on $A$ by $x \mapsto -x$. Then show that $\hat{H^{0}}(H,A) \neq 0$, where $H$ is the subgroup of order 3.
Hope that helps.
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$\begingroup$ But H is a summand of G,does H^0(G,A)=0?About the operate on A? $\endgroup$ Feb 25, 2012 at 7:06