Lie Groups Beyond an Introduction
《Lie Groups Beyond an Introduction》 · 第 35 页 · Chapter I，Section 3「Field Extensions and the Killing Form」，公式 (1.16) 所在定义句
Similarly when a complex Lie algebra $\mathfrak g$ and a real Lie algebra $\mathfrak g_0$ are related as vector spaces over $\mathbb R$ by $\mathfrak g^{\mathbb R}=\mathfrak g_0\oplus i\mathfrak g_0$, we say that $\mathfrak g_0$ is a real form of the complex Lie algebra $\mathfrak g$.