Zeros of Dirichlet series with periodic coefficients

Let $a=(a_n)_{n\ge 1}$ be a periodic sequence, $F_a(s)$ the meromorphic continuation of $\sum_{n\ge 1} a_n/n^s$, and $N_a(σ_1, σ_2, T)$ the number of zeros of $F_a(s)$, counted with their multiplicities, in the rectangle $σ_1 < \Re s < σ_2$, $|\Im s | \le T$. We extend previous results of Laurinčikas, Kaczorowski, Kulas, and Steuding, by showing that if $F_a(s)$ is not of the form $P(s) L_χ (s)$, where $P(s)$ is a Dirichlet polynomial and $L_χ(s)$ a Dirichlet L-function, then there exists an $η=η(a)>0$ such that for all $1/2 < σ_1 < σ_2 < 1+η$, we have $c_1 T \le N_a(σ_1, σ_2, T) \le c_2 T$ for sufficiently large $T$, and suitable positive constants $c_1$ and $c_2$ depending on $a$, $σ_1$, and $σ_2$.

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