Class Field Theory
《Class Field Theory》 · 印刷第 216 页 Section 8.6「Dynkin Diagrams」，Definition 8.31 完整定义
Definition 8.31. If $\Delta=\{\alpha_1,\ldots,\alpha_r\}$ is a base for a root system $R$, the Dynkin diagram for $R$ is a graph having vertices $v_1,\ldots,v_r$. Between two distinct vertices $v_j$ and $v_k$, we place zero, one, two, or three edges according to whether the angle between $\alpha_j$ and $\alpha_k$ is $\pi/2$, $2\pi/3$, $3\pi/4$, or $5\pi/6$. In addition, if $\alpha_j$ and $\alpha_k$ are not orthogonal and have different lengths, we decorate the edges between $v_j$ and $v_k$ with an arrow pointing from the vertex associated to the longer root to the vertex associated to the shorter root.