Lie group valued Koopman eigenfunctions

Abstract

Every continuous-time flow on a topological space has associated to it a Koopman operator, which operates by time-shifts on various spaces of functions, such as C r , L 2, or functions of bounded variation. An eigenfunction of the vector field (and thus for the Koopman operator) can be viewed as an S 1-valued function, which also plays the role of a semiconjugacy to a rigid rotation on S 1. This notion of Koopman eigenfunctions will be generalized to Lie-group valued eigenfunctions, and we will discuss the dynamical aspects of these functions. One of the tools that will be developed to aid the discussion, is a concept of exterior derivative for Lie group valued functions, which generalizes the notion of the differential of a real valued function f. The extended notion of Koopman eigenfunctions utilizes a geometric property of usual eigenfunctions. We show that the generalization in a geometric sense can be used to reveal fundamental properties of usual Koopman eigenfunctions, such as their behaviour under time-rescaling, and as submersions.