Naive Lie Theory
《Naive Lie Theory》 · 第 112 页 · Chapter 6，Section 6.4.14
This is the radical $R(G)$ of $G$. Similarly, there is a maximal closed, connected, unipotent, subgroup of $G$, the unipotent radical $R_u(G)$. We have $R_u(G) = R(G)_u$. $G$ is semi-simple if $R(G) = \{e\}$ and reductive if $R_u(G) = \{e\}$.