Lie Groups Beyond an Introduction
《Lie Groups Beyond an Introduction》 · 第 355 页 · Chapter VI，Section 2「Cartan Decomposition on the Lie Algebra Level」，Killing 型约定及公式 (6.13) 所在定义句（省略中间内容）
Let $B$ be the Killing form. […] An involution $\theta$ of a real semisimple Lie algebra $\mathfrak g_0$ such that the symmetric bilinear form $B_\theta(X,Y)=-B(X,\theta Y)$ is positive definite is called a Cartan involution.