Number Fields
《Number Fields》 · 第 126 页 · Chapter 6「The Distribution of Ideals in a Number Ring」，Exercise 10
Fix a nonzero ideal $M$ in $R=A\cap K$ and define a relation on the set of ideals of $R$ which are relatively prime to $M$: $I\sim_M J$ iff $I=\beta J$ for some $\beta\in R^+$, $\beta\equiv1\pmod M$. […] The equivalence classes are called ray classes.