Dimension drop in residual chains

Abstract: We give a description of the Linnell division ring of a countable residually (poly-$\mathbb Z$ virtually nilpotent) (RPVN) group in terms of a generalised Novikov ring, and show that vanishing top-degree cohomology of a finite type group $G$ with coefficients in this Novikov ring implies the existence of a normal subgroup $N \leqslant G$ such that $\mathrm{cd}{\mathbb Q}(N) < \mathrm{cd}{\mathbb Q}(G)$ and $G/N$ is poly-$\mathbb Z$ virtually nilpotent. As a consequence, we show that if $G$ is an RPVN group of finite type, then its top-degree $\ell^2$-Betti number vanishes if and only if there is a poly-$\mathbb Z$ virtually nilpotent quotient $G/N$ such that $\mathrm{cd}{\mathbb Q}(N) < \mathrm{cd}{\mathbb Q}(G)$. In particular, finitely generated RPVN groups of cohomological dimension $2$ are virtually free-by-nilpotent if and only if their second $\ell^2$-Betti number vanishes, and therefore $2$-dimensional RPVN groups with vanishing second $\ell^2$-Betti number are coherent. As another application, we show that if $G$ is a finitely generated parafree group with $\mathrm{cd}(G) = 2$, then $G$ satisfies the Parafree Conjecture if and only if the terms of its lower central series are eventually free. Note that the class of RPVN groups contains all finitely generated RFRS groups and all finitely generated residually torsion-free nilpotent groups.