A characterization of the (non-trivial) rigid kernel of the Hanoi Tower Group
self-similar group; branch group; rigid kernel; hanoi tower group; congruence problem.
For branch groups, the problem of congruence subgroups can be divided into finding the branch and rigid heads. It has been shown that most of the widely studied branch groups have a trivial hard core, even those with a non-trivial branch core. The first group whose hard core was proved to be non-trivial was the Tower of Hanoi Group, in 2012 by Bartholdi, Siegenthaler and Zalesskii. This dissertation studies which properties this group has that lead it to have a non-trivial rigid core, through a constructive proof that this core is the Klein Group, as done by Skipper in 2019.