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Zbl 1106.11034
Ferreira, Chelo; López, José L.
Asymptotic expansions of the Hurwitz--Lerch zeta function.
(English)
[J] J. Math. Anal. Appl. 298, No. 1, 210-224 (2004). ISSN 0022-247X

In the paper, a generalization of the asymptotic expansions obtained by {\it M.~Katsurada} [Proc.~Japan Acad. 74, No. 10, 167--170 (1998; Zbl 0937.11035)] and {\it D.~Klusch} [J.~Math. Anal. Appl. 170, No. 2, 513--523 (1992; Zbl 0763.11036)] for the Lipschitz-Lerch zeta function $$R(a, x, s)\equiv\sum_{k=0}^\infty {e^{2k\pi ix}\over (a+k)^s}, \quad s, x, a \in \Bbb C, \quad 1-a\notin \Bbb N, \quad \Im x \geq 0,$$ to the Hurwitz-Lerch zeta function $$\Phi(z, s, a)\equiv\sum_{k=0}^\infty {z^k\over (a+k)^s}, \quad 1-a\notin \Bbb N, \quad \vert z\vert <1,$$ is presented. Note that $\Phi({\text e}^{2\pi i x}, s, a)=R(a, x, s)$. First, using an integral formula for the Hurwitz-Lerch zeta function $$\Phi(z, s, a)={1\over \Gamma(s)}\int_0^\infty {x^{s-1}e ^{-ax}\over 1-ze^{-x}}\,d x, \quad \Re a>0, \quad \Re s>0, \quad z\notin[1, \infty),$$ given in [{\it H.~M.~Srivastava} and {\it J.~Choi}, Series associated with the zeta and related functions. Dordrecht: Kluwer Academic Publishers (2001; Zbl 1014.33001)], the authors obtain an integral representation which gives the analytical continuation of the function $\Phi(z, s, a)$ to the region $z\in\Bbb C\setminus[1, \infty)$ if $\Re a>0$, and $z\in\{z\in \Bbb C, \vert z\vert <1\}$ if $\Re a \leq 0$, $a\in \Bbb C\setminus\Bbb R^-$. From this they deduce three complete asymptotic expansions for either large or small $a$ and large $z$ with error bounds. Moreover, the numerical examples for these bounds are presented.
[Renata Macaitiene (Vilnius)]
MSC 2000:
*11M35 Other zeta functions
30D10 Representations of entire functions by series and integrals

Keywords: Hurwitz-Lerch zeta function; Analytic continuation; Asymptotic expansions.

Citations: Zbl 0937.11035; Zbl 0763.11036; Zbl 1014.33001

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