A panaroma of the fundamental group of the modular orbifold
  We give an overview of the category of subgroups of the modular group, incorporating both the tame part, i.e. finite index subgroups, and the non-tame part, i.e. the rest. We also discuss arithmetic related questions which exist in both the tame part (via Belyi's theorem) and the non-tame part. 
A. Muhammed Uluda\u{g} and Ayberk Zeytin
math.AG math.NT