Hecke and Sturm bounds for Hilbert modular forms over real quadratic fields

Abstract

In this article we give an analogue of Hecke and Sturm bounds for Hilbert modular forms over real quadratic fields. Let $K$ be a real quadratic field and $\Om_K$ its ring of integers. Let $\Gamma$ be a congruence subgroup of $\SL_2(\Om_K)$ and $M_{(k_1,k_2)}(\Gamma)$ the space of Hilbert modular forms of weight $(k_1,k_2)$ for $\Gamma$. The first main result is an algorithm to construct a finite set $S$, depending on $K$, $\Gamma$ and $(k_1,k_2)$, such that if the Fourier expansion coefficients of a form $G \in M_{(k_1,k_2)}(\Gamma)$ vanish on the set $S$, then $G$ is the zero form. The second result corresponds to the same statement in the Sturm case, i.e. suppose that all the Fourier coefficients of the form $G$ lie in a finite extension of $\Q$, and let $\id{p}$ be a prime ideal in such extension, whose norm is unramified in $K$; suppose furthermore that the Fourier expansion coefficients of $G$ lie in the ideal $\id{p}$ for all the elements in $S$, then they all lie in the ideal $\id{p}$.