Differencing Methods for Korobov-type exponential sums

TL;DR

This paper introduces a new differencing technique for Korobov-type exponential sums, providing explicit bounds that improve understanding of their behavior and applications to normal numbers and digit analysis.

Contribution

It develops a novel differencing method similar to Weyl-van der Corput, extending bounds for exponential sums and enabling new applications in number theory.

Findings

New explicit bounds for exponential sums around N ≈ exp(log m / log log m)

Enhanced understanding of the sum-product phenomenon in exponential sums

Applications to digits of rational numbers and normal number constructions

Abstract

We study exponential sums of the form $\sum_{n=1}^N e^{2\pi i a b^n/m}$ for non-zero integers $a,b,m$. Classically, non-trivial bounds were known for $N\ge \sqrt{m}$ by Korobov, and this range has been extended significantly by Bourgain as a result of his and others' work on the sum-product phenomenon. We use a new technique, similar to the Weyl-van der Corput method of differencing, to give more explicit bounds bounds that become non-trivial around the time when $\exp(\log m/\log_2\log m) \le N$. We include applications to the digits of rational numbers and constructions of normal numbers.