Number Fields
《Number Fields》 · 第 66 页 · Chapter 4「The Different and Discriminant」
Now consider the ground field to be $\mathbb Q$, so that $R^*=\{\alpha\in K:T_K(\alpha R)\subset\mathbb Z\}$ and $\operatorname{diff}R=\operatorname{diff}(R\mid\mathbb Z)=(R^*)^{-1}$. This is called the absolute different of $R$.