Determinantal hypersurfaces and representations of Coxeter groups

Abstract

Given a finite generating set $T=\{g_0,\dots, g_n\}$ of a group $G$, and a representation $\rho$ of $G$ on a Hilbert space $V$, we investigate how the geometry of the set $D(T,\rho)=\{ [x_0 : \dots : x_n] \in\mathbb C\mathbb P^n \mid \sum x_i\rho(g_i) \text{ not invertible} \}$ reflects the properties of $\rho$. When $V$ is finite-dimensional this is an algebraic hypersurface in $\mathbb C\mathbb P^n$. In the special case $T=G$ and $\rho=$ the left regular representation of $G$, this hypersurface is defined by the \emph{group determinant}, an object studied extensively in the founding work of Frobenius that lead to the creation of representation theory. We focus on the classic case when $G$ is a finite Coxeter group, and make $T$ by adding the identity element $1_G$ to a Coxeter generating set for $G$. Under these assumptions we show in our first main result that if $\rho$ is the left regular representation, then $D(T,\rho)$ determines the isomorphism class of $G$. Our second main result is that if $G$ is not of exceptional type, and $\rho$ is any finite dimensional representation, then $D(T,\rho)$ determines $\rho$.