Existence and uniqueness of global Koopman eigenfunctions for stable fixed points and periodic orbits

TL;DR

This paper establishes existence and uniqueness results for Koopman eigenfunctions in stable dynamical systems, generalizing classical linearization theorems and providing a comprehensive classification of smooth eigenfunctions.

Contribution

It extends Sternberg's linearization theorem to Koopman eigenfunctions, introduces intrinsic definitions of principal eigenfunctions, and proves new uniqueness results for isostable coordinates.

Findings

Proves existence and uniqueness of $C^k$ Koopman eigenfunctions for hyperbolic fixed points and periodic orbits.

Classifies $C^ abla$ eigenfunctions for $C^ abla$ systems with nonresonant linearization.

Shows the uniqueness of the pullback algebra under certain conditions.

Abstract

We consider $C^1$ dynamical systems having an attracting hyperbolic fixed point or periodic orbit and prove existence and uniqueness results for $C^k$ (actually $C^{k,\alpha}_{\text{loc}}$) linearizing semiconjugacies -- of which Koopman eigenfunctions are a special case -- defined on the entire basin of attraction. Our main results both generalize and sharpen Sternberg's $C^k$ linearization theorem for hyperbolic sinks, and in particular our corollaries include uniqueness statements for Sternberg linearizations and Floquet normal forms. Using our main results we also prove new existence and uniqueness statements for $C^k$ Koopman eigenfunctions, including a complete classification of $C^\infty$ eigenfunctions assuming a $C^\infty$ dynamical system with semisimple and nonresonant linearization. We give an intrinsic definition of "principal Koopman eigenfunctions" which generalizes the definition of Mohr and Mezi\'{c} for linear systems, and which includes the notions of "isostables" and "isostable coordinates" appearing in work by Ermentrout, Mauroy, Mezi\'{c}, Moehlis, Wilson, and others. Our main results yield existence and uniqueness theorems for the principal eigenfunctions and isostable coordinates and also show, e.g., that the (a priori non-unique) "pullback algebra" defined in \cite{mohr2016koopman} is unique under certain conditions. We also discuss the limit used to define the "faster" isostable coordinates in \cite{wilson2018greater,monga2019phase} in light of our main results.