#### Vol. 7, No. 4, 2013

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An analogue of Sturm's theorem for Hilbert modular forms

### Yuuki Takai

Vol. 7 (2013), No. 4, 1001–1018
##### Abstract

In this paper, we consider congruences of Hilbert modular forms. Sturm showed that mod $\ell$ elliptic modular forms of weight $k$ and level ${\Gamma }_{1}\left(N\right)$ are determined by the first $\left(k∕12\right)\left[{\Gamma }_{1}\left(1\right):{\Gamma }_{1}\left(N\right)\right]\phantom{\rule{0.2em}{0ex}}mod\phantom{\rule{0.2em}{0ex}}\ell$ Fourier coefficients. We prove an analogue of Sturm’s result for Hilbert modular forms associated to totally real number fields. The proof uses the positivity of ample line bundles on toroidal compactifications of Hilbert modular varieties.

##### Keywords
Hilbert modular forms and varieties, congruences of modular forms, Sturm's theorem, toroidal and minimal compactifications, intersection numbers
##### Mathematical Subject Classification 2010
Primary: 11F41
Secondary: 11F30, 11F33, 14C17