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LSpice
LSpice
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I was reading Brown's Cohomology Theory of Finite groups and was wondering whether there's an example of the following.

Let $n\in \mathbb{N}$ does. Does there exist a $\mathbb{Z}_n$ module $M$ which is cohomologically trivial (i.e. $\hat{H}^i(\mathbb{Z}_n,M)$$\smash{\hat{H}}^i(\mathbb{Z}_n,M)$ is trivial for each $i$) but such that there exists some $p,i\in \mathbb{N}$ such that $\hat{H}^i(\mathbb{Z}_n,M/pM)$$\smash{\hat{H}}^i(\mathbb{Z}_n,M/pM)$ is non-trivial?

In the case that $M$ has no $n$ torsion this is not possible by Lemma 8.6 of Brown's Cohomology of groups, I was wondering what goes wrong in the sense of a counterexample when $M$ has $n$ torsion.

I was reading Brown's Cohomology Theory of Finite groups and was wondering whether there's an example of the following.

Let $n\in \mathbb{N}$ does there exist a $\mathbb{Z}_n$ module $M$ which is cohomologically trivial (i.e. $\hat{H}^i(\mathbb{Z}_n,M)$ is trivial for each $i$) but such that there exists some $p,i\in \mathbb{N}$ such that $\hat{H}^i(\mathbb{Z}_n,M/pM)$ is non-trivial?

In the case that $M$ has no $n$ torsion this is not possible by Lemma 8.6 of Brown's Cohomology of groups, I was wondering what goes wrong in the sense of a counterexample when $M$ has $n$ torsion.

I was reading Brown's Cohomology Theory of Finite groups and was wondering whether there's an example of the following.

Let $n\in \mathbb{N}$. Does there exist a $\mathbb{Z}_n$ module $M$ which is cohomologically trivial (i.e. $\smash{\hat{H}}^i(\mathbb{Z}_n,M)$ is trivial for each $i$) but such that there exists some $p,i\in \mathbb{N}$ such that $\smash{\hat{H}}^i(\mathbb{Z}_n,M/pM)$ is non-trivial?

In the case that $M$ has no $n$ torsion this is not possible by Lemma 8.6 of Brown's Cohomology of groups, I was wondering what goes wrong in the sense of a counterexample when $M$ has $n$ torsion.

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sirjoe
sirjoe
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Cohomologically trivial module $M$ such that $M/pM$ is not cohomologically trivial for some $p\in\mathbb{N}$

I was reading Brown's Cohomology Theory of Finite groups and was wondering whether there's an example of the following.

Let $n\in \mathbb{N}$ does there exist a $\mathbb{Z}_n$ module $M$ which is cohomologically trivial (i.e. $\hat{H}^i(\mathbb{Z}_n,M)$ is trivial for each $i$) but such that there exists some $p,i\in \mathbb{N}$ such that $\hat{H}^i(\mathbb{Z}_n,M/pM)$ is non-trivial?

In the case that $M$ has no $n$ torsion this is not possible by Lemma 8.6 of Brown's Cohomology of groups, I was wondering what goes wrong in the sense of a counterexample when $M$ has $n$ torsion.