On multiplicative functions which are small on average and zero free regions for the Riemann zeta function

Abstract

In this short note we prove the following result: If a completely multiplicative function $f:\mathbb{N}\to [-1,1]$ is small on average in the sense that ${\sum }_{n\le x}f(n)\ll x^{1-\delta }$, for some $\delta >0$, and if the Dirichlet series of $f$, say $F(s)$, is such that $F(1)=0$, then we obtain that for any $\epsilon >0$, ${\sum }_{p\le x}(1+f(p))\log p\ll x^{1-\delta +\epsilon }$. Moreover, a necessary condition for the existence of such $f$ is that the Riemann zeta function $\zeta (s)$ has no zeros in the half plane $Re(s)>1-\delta$.

5 pages, to appear in Lith. Math. J