From telescopes to frames and simple groups

Abstract

We introduce the notion of a telescope of groups. Very roughly a telescope is a directed system of groups that contains various commuting images of some fixed group $B$. Telescopes are inspired from the theory of groups acting on rooted trees. Imitating known constructions of branch groups, we obtain a number of examples of $B$-telescopes and discuss several applications. We give examples of $2$-generated infinite amenable simple groups. We show that every finitely generated residually finite (amenable) group embeds into a finitely generated (amenable) $\mathrm{LEF}$ simple group. We construct $2$-generated frames in products of finite simple groups and show that there are Grothendieck pairs consisting of amenable groups and groups with property $(\tau )$. We give examples of automorphisms of finitely generated, residually finite, amenable groups that are not inner, but become inner in the profinite completion. We describe non-elementary amenable examples of finitely generated, residually finite groups all of whose finitely generated subnormal subgroups are direct factors.