A footnote to a theorem of Halász

Abstract

We study multiplicative functions $f$ satisfying $|f(n)|\le 1$ for all $n$, the associated Dirichlet series $F(s):={\sum }_{n=1}^{\infty }f(n)n^{-s}$, and the summatory function $S_f(x):={\sum }_{n\le x}f(n)$. Up to a possible trivial contribution from the numbers $f(2^k)$, $F(s)$ may have at most one zero or one pole on the one-line, in a sense made precise by Halász. We estimate $\log F(s)$ away from any such point and show that if $F(s)$ has a zero on the one-line in the sense of Halász, then $|S_f(x)|\le (x/\log x)\exp \left(c\sqrt{\log \log x}\right)$ for all $c>0$ when $x$ is large enough. This bound is best possible.

This note is accepted for publication in Functiones et Approximatio