Irreducible finite-index representations of automorphism groups of trees

Abstract: We consider a locally finite tree T where every vertex has degree >= 3, and a closed noncompact subgroup G of the automorphism group Aut(T), with Tits' independence property (P) and with transitive action on the boundary of the tree. We show that G does not have continuous irreducible representations to a locally convex topological vector space over R, C or H that preserve a continuous nondegenerate sesquilinear form of index p > 1. As the representations of index 1 are already classified, this gives a complete classification of continuous irreducible representations of G that preserve a continuous nondegenerate sesquilinear form of finite index.