The $R_{\infty}$ property for infra-nilmanifolds

Abstract

In this paper, we investigate the finiteness of the Reidemeister number $R(f)$ of a selfmap $f\colon M\to M$ on an infra-nilmanifold $M$. We show that the Reidemeister number of an Anosov diffeomorphism on an infra-nilmanifold is always finite. A manifold $M$ is said to have the $R_\infty$ property if $R(f)=\infty$ for every homeomorphism $f\colon M\to M$. We show that every non-orientable generalised Hantzsche-Wendt manifold has the $R_\infty$ property. For an orientable Hantzsche-Wendt manifold $M$, we formulate a criterion, in terms of an associated graph, for $M$ to have the $R_\infty$ property.