Computing actions on cusp forms

For positive integers $k$ and $N$, we describe how to compute the natural action of $SL_2(\mathbb{Z})$ on the space of cusp forms $S_k(Γ(N))$, where a cusp form is given by sufficiently many terms of its $q$-expansion. This will reduce to computing the action of the Atkin--Lehner operator on $S_k(Γ)$ for a congruence subgroup $Γ_1(N)\subseteq Γ\subseteq Γ_0(N)$. Our motivating application of such fundamental computations is to compute explicit models of some modular curves $X_G$.

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