Stabilizers in Coxeter groups with a transpositional action

Abstract

We study simply-laced Coxeter groups whose Coxeter diagram is the line graph $\mathcal{L}Γ$ of some simple graph $Γ$; such groups act on the set of vertices of the graph by transpositions. Our goal is to describe the point stabilizer of this action in detail. For any tree $Σ$, we show that the stabilizer is a finite-index reflection subgroup, and identify its Coxeter diagram, thereby discovering finite-index embeddings of Coxeter groups $W(\mathcal{L}^{[2]}Σ) \hookrightarrow W(\mathcal{L}Σ)$ for any tree $Σ$, including infinitely many cases where the stabilizer itself is simply-laced.