Cusps, Congruence Groups and Monstrous Dessins

Valdo Tatitscheff, Yang-Hui He, John McKay

We study general properties of the dessins d'enfants associated with the Hecke congruence subgroups ${\Gamma }_0(N)$ of the modular group ${\mathrm{PSL}}_2(\mathbb{R})$. The definition of th…