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Let $G$ be a finite group and $G^{\prime}$ be its commutator subgroup. Let $\mathbb{Z}$ and $\mathbb{Q}$ denote the integers and rationals. $\mathbb{Z}$ and $\mathbb{Q/Z}$ treated as trivial $G$-module. We know that $H^1(G,\mathbb{Z})Hom(G,\mathbb{Q/Z}) \cong Hom(G/G^{\prime},\mathbb{Q/Z}) \cong G/G^{\prime} \cong H_1(G,\mathbb{Z})$ and $H^2(G,\mathbb{Q/Z}) \cong H_2(G,\mathbb{Z})$, where $H_n$ and $H^n$ denote the homolgy and cohomology resp.

What are finite groups $H$ such that $H^n(H,\mathbb{Q/Z}) \cong H_n(G,\mathbb{Z})$ ?

I thought Universal coefficient theorem will help. But it do not seem to help.

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    $\begingroup$ What is the relationship between the groups H and G appearing in the question? I guess they're the same? $\endgroup$
    – Dan Ramras
    Commented Apr 16, 2014 at 14:39

1 Answer 1

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It's true for any finite group (for $n>0$).

The long exact sequence of cohomology for the short exact sequence $0\to\mathbb{Z}\to\mathbb{Q}\to\mathbb{Q}/\mathbb{Z}\to0$ gives $H^n(G,\mathbb{Q}/\mathbb{Z})\cong H^{n+1}(G,\mathbb{Z})$, and the Universal Coefficient Theorem gives $\operatorname{Ext}^1(H_n(G,\mathbb{Z}),\mathbb{Z})\cong H^{n+1}(G,\mathbb{Z})$. Finally, $H_n(G,\mathbb{Z})$ is a finite abelian group, and $A\cong\operatorname{Ext}^1(A,\mathbb{Z})$ (non-naturally) for any finite abelian group $A$.

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  • $\begingroup$ There is a shift in degree, though. $\endgroup$ Commented Apr 16, 2014 at 14:08
  • $\begingroup$ What shift in degree? $\endgroup$
    – abx
    Commented Apr 16, 2014 at 14:19
  • $\begingroup$ The question wants an isomorphism of a cohomology group and a homology group in the same degree (I would guess the OP did not write what he intended, in any case) $\endgroup$ Commented Apr 16, 2014 at 14:21
  • $\begingroup$ But that's what Jeremy shows. Of course the isomorphism is not canonical. $\endgroup$
    – abx
    Commented Apr 16, 2014 at 14:41
  • $\begingroup$ There are two shifts in degree that cancel out. $\endgroup$ Commented Apr 16, 2014 at 14:43

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