Bieri-Eckmann Criteria for Profinite Groups

Abstract: In this paper we derive necessary and sufficient homological and cohomological conditions for profinite groups and modules to be of type $\operatorname{FP}n$ over a profinite ring $R$, analogous to the Bieri-Eckmann criteria for abstract groups. We use these to prove that the class of groups of type $\operatorname{FP}_n$ is closed under extensions, quotients by subgroups of type $\operatorname{FP}_n$, proper amalgamated free products and proper $\operatorname{HNN}$-extensions, for each $n$. We show, as a consequence of this, that elementary amenable profinite groups of finite rank are of type $\operatorname{FP}\infty$ over all profinite $R$. For any class $\mathcal{C}$ of finite groups closed under subgroups, quotients and extensions, we also construct pro-$\mathcal{C}$ groups of type $\operatorname{FP}n$ but not of type $\operatorname{FP}{n+1}$ over $\mathbb{Z}_{\hat{\mathcal{C}}}$ for each $n$. Finally, we show that the natural analogue of the usual condition measuring when pro-$p$ groups are of type $\operatorname{FP}_n$ fails for general profinite groups, answering in the negative the profinite analogue of a question of Kropholler.