Algebraic Number Theory
《Algebraic Number Theory》 · 第 361 页 · Section 11.1「A zero-free region」，Theorem 11.3 后
The region $R_q=\{s:\sigma>1-c/\log q\tau\}$ contains no zero of $L(s,\chi)$ unless $\chi$ is a quadratic character, in which case $L(s,\chi)$ has at most one, necessarily real, zero $\beta<1$ in $R_q$. A zero lying in $R_q$, as described above, is called exceptional.