Class Field Theory
《Class Field Theory》 · 第 383 页 · Chapter 13，Section 13.5「Regular Elements」，Definition 13.22
Definition 13.22. If $x \in K$ is contained in a unique maximal torus, $x$ is regular; if $x$ is contained in two distinct maximal tori, $x$ is singular. The set of regular elements in $K$ is denoted $K_{\mathrm{reg}}$ and the set of singular elements is denoted $K_{\mathrm{sing}}$.