Torsion-free nilpotent groups of small Hirsch length with isomorphic finite quotients

Abstract

Let $\mathcal{T}$ denote the class of finitely generated torsion-free nilpotent groups. For a group $G$ let $F(G)$ be the set of isomorphism classes of finite quotients of $G$. Pickel proved that if $G\in \mathcal{T}$, then the set $\mathfrak{g}(G)$ of isomorphism classes of groups $H\in \mathcal{T}$ with $F(G)=F(H)$ is finite. We give an explicit description of the sets $\mathfrak{g}(G)$ for the $\mathcal{T}$-groups $G$ of Hirsch length at most $5$. Based on this, we show that for each Hirsch length $n\ge 4$ and for each $m\in \mathbb{N}$ there is a $\mathcal{T}$-group $G$ of Hirsch length $n$ with $|\mathfrak{g}(G)|\ge m$.

14 pages, preprint version of an accepted article to be published in the Journal of Algebra