Introduction to Lie Algebras and Representation Theory
《Introduction to Lie Algebras and Representation Theory》 · 第 80 页 · Chapter 15，Section 15.3「Cartan Subalgebras」
As a corollary of this proof, we notice that a maximal toral subalgebra of a semisimple Lie algebra (char $F = 0$) has the form $C_L(s)$ for some semisimple element $s$ (cf. Exercise 8.7). Such an element $s$ is called regular semisimple.