1
$\begingroup$

I would like to ask the following: if for a group $G$ the homology $H_n(G,\mathbb{Z})$ is $\mathbb{Z}$-torsion for every $n\geq n_0$, then what can be said concerning $\mathbb{Z}$-torsion for $H_k(G,M)$ where $M$ is a $\mathbb{Z}G$-module? For example I know that if $M$ is a trivial $G$-module, then $$H_n(G,M)\simeq H_n(G,\mathbb{Z})\otimes_{\mathbb{Z}}M\oplus \operatorname{Tor}_1^{\mathbb{Z}}(H_{n-1}(G,\mathbb{Z}),M)$$ [Weibel, Th. 6.1.12] and hence $H_n(G,M)$ is $\mathbb{Z}$-torsion if $H_n(G,\mathbb{Z})$ is. What happens if $M$ is not a trivial $G$-module?

Weibel, "An introduction to homological Algebra"

$\endgroup$
0

2 Answers 2

3
$\begingroup$

There exists an acyclic group $G$ which has the property that there exists a finite-index normal subgroup $H\lhd G$ such that $\mathbb{Z} \leq H_1(H;\mathbb{Z})$. In particular, then $H_1(G;\mathbb{Z}[G/H]) \cong H_1(H;\mathbb{Z})$ is not torsion (by Shapiro's lemma).

The example is the fundamental group of the complement of a wild arc. Examples in the linked paper, such as "Fox's stitch", are infinite cyclic covers of a hyperbolic 2-component link complement. There are such examples which are arithmetic, and therefore have a finite-index subgroup which is "RFRS", by a theorem of mine, in Criteria for virtual fibering. Thus, for any element in the group, there is a finite-index subgroup for which it is homologically non-trivial. This property passes to subgroups, in which case the fundamental group of the complement of Fox's stitch has the property that there is a finite-index subgroup with infinite abelianization (in fact, a non-trivial homomorphism to $\mathbb{Z}$).

$\endgroup$
3
  • $\begingroup$ I guess the OP was primarily interested in $H_k(G;M)$ for $k >> n_0$ (as his universal coefficient example suggests). $\endgroup$
    – Ralph
    Oct 13, 2011 at 8:22
  • $\begingroup$ Your first link is broken, @Ian... $\endgroup$
    – David Roberts
    Mar 30, 2022 at 10:41
  • $\begingroup$ @DavidRoberts thanks, fixed $\endgroup$
    – Ian Agol
    Mar 30, 2022 at 13:51
1
$\begingroup$

This Kunneth formula still holds, I proved it here:

Kuenneth-formula for group cohomology with nontrivial action on the coefficient

which holds in our situation for all nontrivial $G$-modules.

$\endgroup$
1
  • $\begingroup$ Actually, you cannot use this to get your Universal Coefficients Formula unless the actions are trivial! So nevermind. $\endgroup$ Nov 14, 2011 at 20:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.