Commutators, commensurators, and ${\mathrm{PSL}}_2(\mathbb{Z})$

Thomas Koberda, Mahan Mj

Let $H<{\mathrm{PSL}}_2(\mathbb{Z})$ be a finite index normal subgroup which is contained in a principal congruence subgroup, and let $\Phi (H)\ne H$ denote a term of the lower central…