On the Weak Right Order of a Right-Angled Coxeter System

Abstract

Let $(W,S)$ be a Coxeter system, and let $w\in W$. Let $[1,w]:=\{x\in W\mid x{\le }_{R}w\}$ where ${\le }_R$ denotes the weak right order of $(W,S)$. The element $w$ is said to have the \emph{ancestor property} if there is a unique non-trivial involution of maximal length in the set $[1,w]$. The ancestor property was first defined by Hart and Rowley in \cite{hart2025noteinvolutionprefixescoxeter} where they conjectured that all non-identity elements in a finite Coxeter system have the ancestor property. In an arbitrary Coxeter system $(W,S)$, we show that the ancestor property holds for any non-identity fully commutative element (see \cite{stembridge1996fully} for the definition of a fully commutative element). In particular, since any element of a right-angled Coxeter system is fully commutative, we show that the ancestor property holds for all non-identity elements of a right-angled Coxeter system. Lastly, we also provide an axiomatization of right-angled Coxeter systems as reflection systems with a reflection cocycle that obeys a certain property called the \emph{meet intersection condition}.