A note on normal generation and the first ${\ell }^2$-betti number

Abstract

In $2011$, Osin and Thom conjectured that the first ${\ell }^2$-Betti number of a torsion-free discrete group is bounded above by the normal rank of the group minus one. The conjecture has surprising consequences for some fundamental problems in group theory and topology. These include the Wiegold problem on perfect groups, the Levin conjecture, the torsion-free case of the Kervaire conjecture, and an important special case of the Whitehead asphericity conjecture. In this article, we construct for each $n\in \mathbb{N}$ a countable torsion-free group ${\Gamma }_n$ such that ${\beta }_1^{(2)}({\Gamma }_n)=n$ and so that the normal rank, $n({\Gamma }_n)$, equals one. This disproves the conjecture. Our counterexamples are locally free and hence locally indicable. However, they are not finitely generated.

8 pages