Normalisers of subgroups of the modular group (Q1604384)

Let \(G\) be a subgroup of finite index of the modular group \(\Gamma=\text{PSL}_2(\mathbb{Z})\). By using some results due to Kulkarni and Conway, the author givs an algorithm that determines the normalizer of \(G\) in \(\text{PSL}_2(\mathbb{Z})\). Since \(N(G)/G\) is a subgroup of the automorphism group of the compact Riemann surface \(H^*/G\), we agree with the author that this work can be considered as the first step towards the determination of the automorphism group of \(H^*/G\).