Wohlfahrt's Theorem for the Hecke group G_5

Abstract

Let K be a subgroup of the inhomogeneous Hecke group G_5 of finite index. Suppose that the geometric level of K is r. Then K is congruence if and only if K contains the principal congruence subgroup of level 2r.

Cited by in corpus (2)

• Identifying normal and congruence subgroups
• Wohlfahrt's Theorem and index formula for elementary matrix groups and $SL(2,\mathcal{O})$