Introduction to Lie Algebras and Representation Theory
《Introduction to Lie Algebras and Representation Theory》 · 印刷第 15 页 Chapter II 章首域的约定、第 83 页 Section 16.3「Borel subalgebras」开头定义句（省略中间内容）
Virtually all of the remaining theory to be developed in this book will require the assumption that $F$ have characteristic $0$. (Some of the exercises will indicate how counterexamples arise in prime characteristic.) Moreover, in order to have available the eigenvalues of $\operatorname{ad}x$ for arbitrary $x$ (not just for $\operatorname{ad}x$ nilpotent), we shall assume that $F$ is algebraically closed, except where otherwise specified. […] To pass from the solvable case to the general case, we utilize Borel subalgebras of a Lie algebra $L$, which are by definition the maximal solvable subalgebras of $L$.