Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 16 页 · Section 1.3「Subgroups and Cosets」，小节「Normal Closure and Core」
If $X$ is a nonempty subset of a group $G$, the normal closure of $X$ in $G$ is the intersection of all the normal subgroups of $G$ which contain $X$.