The Weyl group of type $A_1$ root systems extended by an abelian group

Abstract

We investigate the class of root systems $R$ obtained by extending an $A_1$-type irreducible root system by a free abelian group $G$. In this context there is a Weyl group $W$ and a group $U$ with the presentation by conjugation. Both groups are reflection groups with respect to a discrete symmetric space $T$ associated to $R$. We show that the natural homomorphism $U\to W$ is an isomorphism if and only if an associated subset $T^{ab}\setminus\{0\}$ of $G_2=G/2G$ is 2-independent, i.e. its image under the map $G_2\to G_2\otimes G_2, g\mapsto g\otimes g$ is linearly independent over the Galois field $F_2$.