Translation lengths of outer automorphisms of finitely generated free-by-finite groups

Abstract

Bestvina, Feighn and Handel proved that every subgroup of the outer automorphism group, of the free group of rank n is either virtually finitely generated abelian or contains a nonabelian free group. In this note we consider the more general situation of the outer automorphism group of a finitely generated free-by-finite group G. We show that is translation discrete and that every subgroup of is either virtually finitely generated abelian or contains a nonabelian free group.