Lie Groups Beyond an Introduction
《Lie Groups Beyond an Introduction》 · 第 32 页 · Chapter I, Section 2「Ideals」，Proposition 1.12 及证明后的命名句（省略证明）
Proposition 1.12. If $\mathfrak g$ is a finite-dimensional Lie algebra, then there exists a unique solvable ideal $\mathfrak r$ of $\mathfrak g$ containing all solvable ideals in $\mathfrak g$. […] The ideal $\mathfrak r$ of Proposition 1.12 is called the radical of $\mathfrak g$ and is denoted $\operatorname{rad}\mathfrak g$.