Convolution of periodic multiplicative functions and the divisor problem

We study a certain class of arithmetic functions that appeared in Klurman's classification of $\pm 1$ multiplicative functions with bounded partial sums, c.f., Comp. Math. 153 (8), 2017, pp. 1622-1657. These functions are periodic and $1$-pretentious. We prove that if $f_1$ and $f_2$ belong to this class, then $\sum_{n\leq x}(f_1\ast f_2)(n)=Ω(x^{1/4})$. This confirms a conjecture by the first author. As a byproduct of our proof, we studied the correlation between $Δ(x)$ and $Δ(θx)$, where $θ$ is a fixed real number. We prove that there is a non-trivial correlation when $θ$ is rational, and a decorrelation when $θ$ is irrational. Moreover, if $θ$ has a finite irrationality measure, then we can make it quantitative this decorrelation in terms of this measure.

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