Faithful Burau-like representations of some rank two Garside groups and torus knot groups

Abstract

We give a method to produce faithful representations of the groups $G(n,m)=\langle X, Y \ \vert \ X^m = Y^n \rangle$ in $\mathrm{GL}_2(\mathbb{C}[t^{\pm 1}, q^{\pm 1}])$. These groups are Garside groups and the Garside normal forms of elements of the corresponding monoid can be explicitly recovered from the matrices, in the spirit of Krammer's proof of the linearity of Artin's braid groups. We use this method to construct several explicit faithful representations of the above groups, among which a representation which generalizes the reduced Burau representation of $B_3 \cong G(2,3)$ to a large family of groups of the form $G(n,m)$ with $n$, $m$ coprime (which are torus knot groups). Like the Burau representation, this representation specializes to a representation of a reflection-like quotient that we previously introduced, called \textit{$2$-toric reflection group}. As a byproduct we get a "Burau representation" for some exceptional complex braid groups, which also shows that the latter embed into their Hecke algebra.