A folklore conjecture on multiplicative exponential sums

Abstract

We prove a folklore conjecture on exponential sums with multiplicative coefficients: if $f$ is multiplicative with $|f(n)|\le1$, $(a,q)=1$, $|α-a/q|\le q^{-2}$ and $q\le N/(\log N)^2$, then \[ \sum_{n\le N}f(n)\operatorname{e}(nα)\ll\frac N{\log N}+\frac N{\sqrt q}. \] Complete multiplicativity is not required. We also prove two related conjectured bounds: the refinement in which $q$ is replaced by $q(1+N|α-a/q|)$, when this quantity is at most $N/(\log N)^2$, and the corresponding bound for the sum with logarithmic weights. Finally, we show that the bound can fail for multiplicative $f$ satisfying only $|f(p)|\le A$ at primes and $\sum_{n\le x}|f(n)|^2\le A^2x$, for every fixed $A>1$.