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The q-expansion of a modular form $f(z)$ is its Fourier expansion at the cusp $z=i\infty$ ($q=0$), expressed as a power series $\sum a_n q^n$ in the variable $q=e^{2\pi iz}$.

For cusp forms the constant coefficient $a_0$ of the $q$-expansion is zero.

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• Review status: reviewed
• Last edited by David Farmer on 2019-04-29 09:28:57
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