2025/06/15 by Pierre-Alexandre Bazin, Pylaiev, Ihor, Ihor Pylaiev +2 · 1 citation
Mathematics · #math.NT
paper · pdf · doi:10.48550/arxiv.2506.12845
We investigate when the exponential sum Sf(x,α) := ∑n≤ xf(n)e(nα) is bounded, for a multiplicative function f and α∈ℝ. We show that under natural assumptions, Sf(x,α) is bounded only when f is very close to a twisted Dirichlet character χ(n)nit. We obtain sharper classification results for functions that are completely multiplicative or take only finitely many values, including a complete classification in the case when f is completely multiplicative and α is irrational. We also prove a stronger classification under the assumption that the sum is bounded for a positive measure set of α.