Fourier expansions at cusps

Abstract: In this article we study the number fields generated by the Fourier coefficients of modular forms at arbitrary cusps. We give upper bounds for these number fields which are cyclotomic extensions of the field generated by the Fourier coefficients at $\infty$, and determine them explicitly for newforms with trivial Nebentypus. The main tool is an extension of a result of Shimura on the compatibility between the actions of $\mathrm{SL}_2(\mathbb{Z})$ and $\mathrm{Aut}(\mathbb{C})$ on the space of modular forms. We give two new proofs of this result: one based on products of Eisenstein series, and the other using the theory of algebraic modular forms.