Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 46 页 · Section 2.1「Free Groups」，小节「Reduced Words」
A word $w$ in $X$ is called reduced if it contains no pair of consecutive symbols of the form $xx^{-1}$ or $x^{-1}x$, $(x\in X)$. By convention the empty word is reduced.