Introduction to Analytic Number Theory
《Introduction to Analytic Number Theory》 · 第 249 页 · Section 12.1「Introduction」
The treatment of both $\zeta(s)$ and $L(s,\chi)$ can be unified by introducing the Hurwitz zeta function $\zeta(s,a)$ defined for $\sigma>1$ by the series $\zeta(s,a)=\sum_{n=0}^{\infty}(n+a)^{-s}$, where $a$ is a fixed real number, $0<a\le1$.