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Consider a profinite group $S$ acting trivially on $\mathbb{F}_p$. Choose $\chi \neq 0\in H^1(S, \mathbb{F}_p)$ and set $T = \ker(\chi)$. Let $X$ be the $S$-Module of all functions $S/T \rightarrow \mathbb{F}_p$.

Show that there exists a canonical isomorphism $ H^q(S,X) \cong H^q(T,\mathbb{F}_p). $

The case $q=0$ is obvious, since $X^S$ consists of the constant functions and $\mathbb{F}_p^T = \mathbb{F}_p$ may be embedded into $X$ as the constant functions.

But how do I proceed from there? Dimension shifting doesn't seem to work here or am I missing something?

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    Yes it does. Thank you for the hint2012-10-05

1 Answers 1

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Thanks to i. m. soloveichik I was able to solve my question.

There exists an isomorphism

$ \text{Ind}^T_S(\mathbb{F}_p) \cong \text{Map}(S/T, \mathbb{F}_p) = X $ given by $ x(\sigma) \mapsto y(\sigma T) = \sigma x (\sigma^{-1}) $

By Shapiro's Lemma we get the isomorphism

$ H^q(S, \: \text{Ind}^T_S(\mathbb{F}_p)) \cong H^q(T,\mathbb{F}_p). $

A proof is found in Neukirch/Schmidt/Wingberg: 'Cohomology of Number Fields' p. 60.