Lie Groups Beyond an Introduction
《Lie Groups Beyond an Introduction》 · 第 45 页 · Chapter I，Section 6「Nilpotent Lie Algebras and Engel's Theorem」
A solvable Lie algebra $\mathfrak g$ is said to be split solvable if there is an elementary sequence $\mathfrak g = \mathfrak a_0 \supseteq \mathfrak a_1 \supseteq \dots \supseteq \mathfrak a_n = 0$ in which each $\mathfrak a_i$ is an ideal in $\mathfrak g$ (rather than just in $\mathfrak a_{i-1}$).