Fake Mu's

Abstract

Let $f(n)$ denote a multiplicative function with range $\{-1,0,1\}$, and let $F(x) = \sum_{n\leq x} f(n)$. Then $F(x)/\sqrt{x} = a\sqrt{x} + b + E(x)$, where $a$ and $b$ are constants and $E(x)$ is an error term that either tends to $0$ in the limit, or is expected to oscillate about $0$ in a roughly balanced manner. We say $F(x)$ has persistent bias $b$ (at the scale of $\sqrt{x}$) in the first case, and apparent bias $b$ in the latter. For example, if $f(n)=\mu(n)$, the M\"{o}bius function, then $F(x) = \sum_{n\leq x} \mu(n)$ has $b=0$ so exhibits no persistent or apparent bias, while if $f(n)=\lambda(n)$, the Liouville function, then $F(x) = \sum_{n\leq x} \lambda(n)$ has apparent bias $b=1/\zeta(1/2)$. We study the bias when $f(p^k)$ is independent of the prime $p$, and call such functions fake $\mu's$. We investigate the conditions required for such a function to exhibit a persistent or apparent bias, determine the functions in this family with maximal and minimal bias of each type, and characterize the functions with no bias of either type. For such a function $F(x)$ with apparent bias $b$, we also show that $F(x)/\sqrt{x}-a\sqrt{x}-b$ changes sign infinitely often.