Class Field Theory
《Class Field Theory》 · 第 204 页 · Section 8.3「Duality」，Definition 8.10 的余根定义
If $(E,R)$ is a root system, then for each root $\alpha\in R$, the coroot $H_\alpha$ is the vector given by $H_\alpha=2\frac{\alpha}{\langle\alpha,\alpha\rangle}$.