Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 12 页 · Section 1.3「Subgroups and Cosets」，小节「The Order of an Element」
A group element $x$ has finite order $n$ if the cyclic subgroup $\langle x\rangle$ has order $n$. If $\langle x\rangle$ is infinite, then $x$ has infinite order. We shall write $|x|$ for the order of $x$. Elements of order 2 are often called involutions.