Fix some $1 \leq k \leq n$. I'm looking for finite-dimensional vector spaces $M\_{n,k}$ over $\mathbb{Q}$ on which $\mathbb{Z}^n$ acts such that the natural map $H\_k(\ell \mathbb{Z}^n,M\_{n,k}) \rightarrow H\_k(\mathbb{Z}^k,M\_{n,k})$ is not an isomorphism for some $\ell \geq 2$. Here $\ell \mathbb{Z}^n$ is the subgroup of $\mathbb{Z}^n$ consisting of vectors each of whose entries is divisible by $\ell$ and the map on group homology is induced by the inclusion $\ell \mathbb{Z}^n \hookrightarrow \mathbb{Z}^n$.



Homology of abelian groups and their finite-index subgroups
$k=n=1$, $M\_{n,k}=M=\mathbb{Q}$. Let $\mathbb{Z}$ act on $\mathbb{Q}$ by $n \cdot q = (-1)^n q$. 


The homology is $H\_i(\mathbb{Z};M)=\mathbb{Q}$ for $i=1$ and $0$ otherwise.
The subgroup $2 \mathbb{Z}$ acts trivially on $M$ and so $H\_i (2 \mathbb{Z};M)=\mathbb{Q}$ for $i=0$ and $0$ otherwise.


