Naive Lie Theory
《Naive Lie Theory》 · 第 179 页 · Section 10.2「Simply laced root systems」，10.2.5，式 (46)
A derivation of the Lie algebra $\mathfrak g$ is a linear map $\Delta$ of $\mathfrak g$ such that for $a,b\in\mathfrak g$ $\Delta[a,b]=[\Delta a,b]+[a,\Delta b]$.