Almost periodic structures and the semiconjugacy problem

TL;DR

This paper investigates when almost periodic dynamical systems can be simplified to rigid translations, providing a general criterion based on boundedness of orbit distances, applicable to various models including quasicrystals and differential equations.

Contribution

It establishes a unifying criterion for semiconjugacy to minimal translations in a broad class of almost periodic flows, extending previous results to new settings.

Findings

Semiconjugacy exists if and only if a boundedness condition is satisfied.

The results apply to scalar differential equations with almost-periodic space and time.

The framework includes skew products, quasicrystals, and real-line maps with almost-periodic displacements.

Abstract

The description of almost periodic or quasiperiodic structures has a long tradition in mathematical physics, in particular since the discovery of quasicrystals in the early 80's. Frequently, the modelling of such structures leads to different types of dynamical systems which include, depending on the concept of quasiperiodicity being considered, skew products over quasiperiodic or almost-periodic base flows, mathematical quasicrystals or maps of the real line with almost-periodic displacement. An important problem in this context is to know whether the considered system is semiconjugate to a rigid translation. We solve this question in a general setting that includes all the above-mentioned examples and also allows to treat scalar differential equations that are almost-periodic both in space and time. To that end, we study a certain class of flows that preserve a one-dimensional foliation and show that a semiconjugacy to a minimal translation flow exists if and only if a boundedness condition, concerning the distance of orbits of the flow to those of the translation, holds.