Modular hyperbolas and Beatty sequences

Abstract

Bounds for $\max\{m,\tilde{m}\}$ subject to $m,\tilde{m} \in \mathbb{Z}\cap[1,p)$, $p$ prime, $z$ indivisible by $p$, $m\tilde{m}\equiv z\bmod p$ and $m$ belonging to some fixed Beatty sequence $\{ \lfloor n\alpha+\beta \rfloor : n\in\mathbb{N} \}$ are obtained, assuming certain conditions on $\alpha$. The proof uses a method due to Banks and Shparlinski. As an intermediate step, bounds for the discrete periodic autocorrelation of the finite sequence $0,\, \operatorname{e}_p(y\overline{1}), \operatorname{e}_p(y\overline{2}), \ldots, \operatorname{e}_p(y(\overline{p-1}))$ on average are obtained, where $\operatorname{e}_p(t) = \exp(2\pi i t/p)$ and $m\overline{m} \equiv 1\bmod p$. The latter is accomplished by adapting a method due to Kloosterman.