Real zeros of Hurwitz-Lerch zeta and Hurwitz-Lerch type of Euler-Zagier double zeta functions

Abstract

Let $0 < a \le 1$, $s,z \in {\mathbb{C}}$ and $0 < |z|\le 1$. Then the Hurwitz-Lerch zeta function is defined by $Φ(s,a,z) := \sum_{n=0}^\infty z^n(n+a)^{-s}$ when $σ:=\Re (s) >1$. In this paper, we show that the Hurwitz zeta function $ζ(σ,a) := Φ(σ,a,1)$ does not vanish for all $0 <σ<1$ if and only if $a \ge 1/2$. Moreover, we prove that $Φ(σ,a,z) \ne 0$ for all $0 <σ<1$ and $0 < a \le 1$ when $z \ne 1$. Real zeros of Hurwitz-Lerch type of Euler-Zagier double zeta functions are studied as well.