Coxeter systems, left inversion sets, and higher dimensional cubes

Abstract

Let $(W,S)$ be a Coxeter system. We investigate the equation $w(\hat{I}{¦}_x)=\hat{I}{¦}_y$ where $w,x,y\in W$ and $\hat{I}{¦}_x$, $\hat{I}{¦}_y$ denote the left inversion sets of $x$ and $y$. We then define a commutative square diagram called a Coxeter square which describes the relationship between 4 non-identity elements of the Coxeter group $W$ and the equation $w(\hat{I}{¦}_x)=\hat{I}{¦}_y$. Coxeter squares were first introduced by Dyer, Wang in \cite{dyer2011groupoids2} and \cite{dyer2019characterization}. Coxeter squares can be \textquotedblleft glued" together by compatible edges to form commutative diagrams in the shape of higher dimensional cubes called Coxeter $n$-cubes, which were first defined by Dyer in Example 12.5 of \cite{dyer2011groupoids2}. When $|W|<\infty$ and $|S|=n$, we show that Coxeter $n$-cubes must exist within $(W,S)$. We then prove results about Coxeter $n$-cubes in the $A_n$ Coxeter system. We establish an explicit bijection between Coxeter $n$-cubes (modulo orientation) in $A_n$ and binary trees with $n+1$ leaves. We also show that an element $x$ of $A_n$ appears as the edge of some Coxeter $n$-cube if and only if $x$ is a bigrassmannian permutation.