Weighted partial sums of a random multiplicative function and their positivity

Abstract

In this paper, we study the probability that some weighted partial sums of a random multiplicative function $f$ are positive. Applying the characteristic decomposition, we obtain that if $S$ is a non-empty subset of the multiplicative residue class group $(\mathbb{Z}/m\mathbb{Z})^{\times}$ with $m$ being a fixed positive integer and $A=\{a+mn\mid n=0,1,2,3,\cdots\}$ with $a\in S,$ then there exists a positive number $δ$ independent of $x$, such that \[ \mathbb{P}\left(\sum_{A\cap[1,x)}\frac{f(n)}{n}<0\right)>δ\] unless the coefficients of the real characters in the expansion of the characteristic function of $S$ according to the characters of $(\mathbb{Z}/m\mathbb{Z})^{\times}$ are all non-negative, and the coefficients of the complex characters are all zero, in which case we have \[ \mathbb{P}\left(\sum_{A\cap[1,x)}\frac{f(n)}{n}<0\right)=O\left(\exp\left(-\exp\left(\frac{\ln x}{C\ln_{2}x}\right)\right)\right) \] for a positive constant $C.$ This includes as a special case a result of Angelo and Xu. We also extend the result to the cyclotomic field $K_{n}=\mathbb{Q}(ζ_{n})$ with $ζ_{n}=e^{2πi/n}$ and study the probability that these generalized weighted sums are positive. In addition, we deal with the positivity problem of certain partial sums related to the celebrated Ramanujan tau function $τ(n)$ and the Ramanujan modular form $Δ(q),$ and obtain an upper bound for the probability that these partial sums are negative in a more general situation.