A First Course in Modular Forms
《A First Course in Modular Forms》 · 第 120–121 页 · Section 4.4「Gamma, zeta, and L-functions」
Every Dirichlet character $\chi$ modulo $N$ has an associated Dirichlet $L$-function similar to the Riemann zeta function, $L(s,\chi)=\sum_{n=1}^{\infty}\frac{\chi(n)}{n^s}=\prod_{p\in\mathcal P}(1-\chi(p)p^{-s})^{-1}$, $\operatorname{Re}(s)>1$.