On nilspace systems and their morphisms

TL;DR

This paper investigates morphisms of nilspace systems, showing they factor through morphisms respecting translations, and strengthens the inverse limit theorem for compact nilspaces, impacting the structure theory of characteristic factors.

Contribution

It introduces a new factorization result for nilspace morphisms and enhances the inverse limit theorem for nilspaces, advancing the understanding of their structure and morphisms.

Findings

Morphisms of nilspace systems can be factored through translation-consistent morphisms.

Strengthened inverse limit theorem for compact nilspaces applicable to nilspace systems.

Implications for the structure theorem of characteristic factors in ergodic theory.

Abstract

A nilspace system is a generalization of a nilsystem, consisting of a compact nilspace X equipped with a group of nilspace translations acting on X. Nilspace systems appear in different guises in several recent works, and this motivates the study of these systems per se as well as their relation to more classical types of systems. In this paper we study morphisms of nilspace systems, i.e., nilspace morphisms with the additional property of being consistent with the actions of the given translations. A nilspace morphism does not necessarily have this property, but one of our main results shows that it factors through some other morphism which does have the property. As an application we obtain a strengthening of the inverse limit theorem for compact nilspaces, valid for nilspace systems. This is used in work of the first and third named authors to generalize the celebrated structure theorem of Host and Kra on characteristic factors.