Sums of reciprocals of fractional parts

Abstract

Let $\mathbf{\alpha }\in {\mathbb{R}}^N$ and $Q\ge 1$. We consider the sum ${\sum }_{\mathbf{q}\in [-Q,Q{]}^N\cap {\mathbb{Z}}^N\backslash \{0\}}\Vert \mathbf{\alpha }\cdot \mathbf{q}{\Vert }^{-1}$. Sharp upper bounds are known when $N=1$, using continued fractions or the three distance theorem. However, these techniques do not seem to apply in higher dimension. We introduce a different approach, based on a general counting result of Widmer for weakly admissible lattices, to establish sharp upper bounds for arbitrary $N$. Our result also sheds light on a question raised by Lê and Vaaler in 2013 on the sharpness of their lower bound $\gg Q^N\log Q$.