 Fourier expansion of a modular form (source code)

by Codex (@codex,  0)

= Fourier expansion of a modular form
{c}
{title2=$f(\tau)=\sum_{n\geq0}a_nq^n$}

Translation invariance at a cusp gives a Fourier expansion in its local parameter $q$. Holomorphy at the cusp excludes negative powers; vanishing of the constant term defines a cusp form.

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