Powers of Coxeter elements in infinite groups are reduced

Abstract:

Let $W$ be an infinite irreducible Coxeter group with $(s_1, \ldots , s_n)$ the simple generators. We give a short proof that the word $s_1 s_2 \cdots s_n s_1 s_2 \cdots$ $s_n \cdots s_1 s_2 \cdots s_n$ is reduced for any number of repetitions of $s_1 s_2 \cdots s_n$. This result was proved for simply laced, crystallographic groups by Kleiner and Pelley using methods from the theory of quiver representations. Our proof uses only basic facts about Coxeter groups and the geometry of root systems. We also prove that, in finite Coxeter groups, there is a reduced word for $w_0$ which is obtained from the semi-infinite word $s_1 s_2 \cdots s_n s_1 s_2 \cdots s_n \cdots$ by interchanging commuting elements and taking a prefix.