Cusp of a modular group
= Cusp of a modular group
A cusp of a finite-index subgroup $\Gamma\leq SL_2(\mathbb Z)$ is an orbit in $\Gamma\backslash\mathbb P^1(\mathbb Q)$. If $\sigma\in SL_2(\mathbb Z)$ sends infinity to a cusp, behavior there is studied through $f|_k\sigma$.