On the size of the Fourier coefficients of Hilbert cusp forms

Abstract

Let $\mathbf{f}$ be a primitive Hilbert cusp form of weight $k$ and level $\mathfrak{n}$ with Fourier coefficients $c_{\mathbf{f}}(\mathfrak{m})$. We prove a non-trivial upper bound for almost all Fourier coefficients $c_{\mathbf{f}}(\mathfrak{m})$ of $\mathbf{f}$. This generalizes the bounds obtained by Luca, Radziwiłł and Shparlinski. We also prove the existence of infinitely many integral ideals $\mathfrak{m}$ for which the Fourier coefficients $c_{\mathbf{f}}(\mathfrak{m})$ have the improved upper bound and further we obtain a refinement of these integral ideals in terms of prime powers. In particular, this enable us to deduce the bound for Fourier coefficients of elliptic cusp forms beyond the `typical size'. Moreover, we prove further improvements of the bound under the assumption of Littlewood's conjecture. Finally, We study a lower bound for the Fourier coefficients at prime powers provided the corresponding Hecke eigen angle is badly approximable.

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