Naive Lie Theory
《Naive Lie Theory》 · 第 21 页 · Chapter 2 章首域的约定及 Section 2.1「Algebraic groups」，2.1.1 的代数群与仿射性条件（省略中间内容）
$k$ is an algebraically closed field and $F$ a subfield. All algebraic varieties are over $k$. […] An algebraic group is an algebraic variety $G$ which is also a group such that the maps defining the group structure $\mu:G\times G\to G$ with $\mu(x,y)=xy$ and $i:x\mapsto x^{-1}$ are morphisms of varieties. […] If the underlying variety is affine, $G$ is a linear algebraic group.