H\"older conditions for endomorphisms of hyperbolic groups

Abstract

It is proved that an endomorphism $\varphi$ of a hyperbolic group $G$ satisfies a H\"older condition with respect to a visual metric if and only if $\varphi$ is virtually injective and $G\varphi$ is a quasiconvex subgroup of $G$. If $G$ is virtually free or torsion-free co-hopfian, then $\varphi$ is uniformly continuous if and only if it it satisfies a H\"older condition if and only if it is virtually injective. Lipschitz conditions are discussed for free group automorphisms.