Commutator subgroup of the Hecke group $G_5$ is not congruence

Abstract

We prove that the power subgroup $G_5^5$ and the commutator subgroup $G_5^{\prime }$ of the Hecke group $G_5$ are not congruence.

arXiv commentLemmas 3.5-3.7 are removed as they are not used in the proof of Prop. 5.2. arXiv admin note: text overlap with arXiv:1401.0775