Algebraic Number Theory
《Algebraic Number Theory》 · 第 542 页 · Chapter VII，Section 13「Prime Distribution」，Definition
A set $S$ of prime ideals of $K$ is said to have Dirichlet density $\delta(S)$ if the limit $\delta(S)=\lim_{s\to1+}\frac{\sum_{\mathfrak p\in S}N\mathfrak p^{-s}}{\log(1/(s-1))}$ exists.