On subgroups of finite index in branch groups

Abstract

We give a structural description of the normal subgroups of subgroups of finite index in branch groups in terms of rigid stabilizers. This gives further insight into the structure lattices of branch groups introduced by the second author. We derive a condition concerning abstract commensurability of branch groups acting on the p-ary tree for any prime p.

arXiv comment9 pages, no figures; minor changes made following referees suggestions, some explanations improved