 Modular form on a finite-index subgroup

A modular form on an arbitrary finite-index subgroup $\Gamma \le S{L}_2(\mathbb{Z})$ is a holomorphic function on the complex upper half-plane, invariant under the weight-$k$ slash operator for modular forms, and holomorphic at a cusp at every cusp of a modular group for $\Gamma$. If $\sigma \in S{L}_2(\mathbb{Z})$ maps infinity to a cusp, choose a genuine translation period $h>0$ with $\sigma T^h{\sigma }^{-1}\in \Gamma$. Then $f{|}_k\sigma$ has a convergent series in $e^{2\pi iz/h}$ with nonnegative exponents. A cusp form has zero constant term at every cusp. This definition allows noncongruence subgroups and avoids sign ambiguities in odd weights when a smaller width is defined only modulo the center.