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Let $G$ be a group (possibly infinite) and $k$ be a field. A module $M$ over $k[G]$ is said to be of type $\text{FP}_{\infty}(k)$ if it has a projective resolution each of whose terms is finitely generated. We say that $G$ itself is of type $\text{FP}_{\infty}(k)$ if the trivial $k[G]$-module $k$ is of type $\text{FP}_{\infty}(k)$.

Assume that $G$ is a group of type $\text{FP}_{\infty}(k)$ and that $M$ is a $k[G]$-module that is finite-dimensional over $k$ (in other words, $M$ is a finite-dimensional representation of $G$). Must $M$ be of type $\text{FP}_{\infty}(k)$? If not, are there stronger finiteness properties that we can put on $G$ to assure that this holds (for instance, having a compact $K(G,1)$)?

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Cleaned up answer (6/11/18) after comments of Andy Putman.

The answer is yes.

Thm 2 of https://www.tandfonline.com/doi/abs/10.1080/00927870600796110 shows that if G is $FP_\infty$ over $k$, then $kG$ has a free resolution as a bimodule by finitely generated free bimodules in each dimension.

If you tensor this resolution with $M$ over $kG$ you get a free resolution of $M$ with the finiteness properties you want. Tensoring with $M$ gives a resolution because its homology is $Tor^{kG}(M,kG)$.

It is easy to check that $(kG\otimes_k kG)\otimes_{kG} M\cong kG^{\dim M}$ as a left $kG$-module so that the free resolution is finitely generated in each degree. The basis as a $kG$-module is the tensors $1\otimes 1\otimes b$ with $b$ running over a basis of $M$.

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  • $\begingroup$ Exactness is preserved because the complex resulting of tensoring by $M$ computes $Tor^{kG}(M,kG)$. $\endgroup$ Commented Mar 9, 2014 at 21:11
  • $\begingroup$ @Mariano, that was the argument in [10]. Thanks! $\endgroup$ Commented Mar 9, 2014 at 21:29
  • $\begingroup$ Even after the edit, are you sure you linked to the paper you wanted to link to? I believe that the result you refer to is Theorem 2 in tandfonline.com/doi/abs/10.1080/00927870600796110 $\endgroup$ Commented Jun 11, 2018 at 20:18
  • $\begingroup$ @AndyPutman, I believe it was already known that bi-FP_n=FP_n for groups when Pride wrote this. I think I got confused the first time as to which paper proved it first. I'm not sure the publication dates of the communications in algebra papers reflect when they are written. $\endgroup$ Commented Jun 11, 2018 at 20:26
  • $\begingroup$ I don't know the history, so I can't say anything there. But I have trouble locating the result you are talking about in Kobayashi-Otto's paper (indeed, it looks like they only talk about monoids, not groups). Can you point out where it is? $\endgroup$ Commented Jun 11, 2018 at 20:36

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