Expanding maps and non-trivial self-covers on infra-nilmanifolds

TL;DR

This paper provides a complete algebraic characterization of infra-nilmanifolds that admit expanding maps or non-trivial self-covers, linking these properties to their rational holonomy representations and co-Hopfian nature.

Contribution

It offers a novel algebraic criterion for the existence of expanding maps and non-trivial self-covers on infra-nilmanifolds based on holonomy and co-Hopfian properties.

Findings

Characterization of infra-nilmanifolds admitting expanding maps.

Criteria for infra-nilmanifolds with non-trivial self-covers.

New examples of infra-nilmanifolds without these features.

Abstract

Every expanding map on a closed manifold is topologically conjugate to an expanding map on an infra-nilmanifold, but not every infra-nilmanifold admits an expanding map. In this article we give a complete algebraic characterization of the infra-nilmanifolds admitting an expanding map. We show that, just as in the case of Anosov diffeomorphisms, the existence of an expanding map depends only on the rational holonomy representation of the infra-nilmanifold. A similar characterization is also given for the infra-nilmanifolds with a non-trivial self-cover, which corresponds to determining which almost-Bieberbach groups are co-Hopfian. These results provide many new examples of infra-nilmanifolds without non-trivial self-covers or expanding maps.