The congruence subgroup problem for branch groups

Abstract

We state and study the congruence subgroup problem for groups acting on rooted tree, and for branch groups in particular. The problem is reduced to the computation of the congruence kernel, which we split into two parts: the branch kernel and the rigid kernel. In the case of regular branch groups, we prove that the first one is Abelian while the second has finite exponent. We also establish some rigidity results concerning these kernels. We work out explicitly known and new examples of non-trivial congruence kernels, describing in each case the group action. The Hanoi tower group receives particular attention due to its surprisingly rich behaviour.

22 pages, no figure

References in corpus (1)

• The Twisted Twin of the Grigorchuk Group

Cited by in corpus (6)

• Locally normal subgroups of totally disconnected groups. Part I: General theory
• Pro-$\mathcal{C}$ congruence properties for groups of rooted tree automorphisms
• On the congruence subgroup problem for branch groups
• Commensurability growth of branch groups
• Finitely generated subgroups of branch groups and subdirect products of just infinite groups
• On the congruence subgroup property for GGS-groups