A Data-Driven Approximation of the Koopman Operator: Extending Dynamic Mode Decomposition

The Koopman operator is a linear but infinite-dimensional operator that governs the evolution of scalar observables defined on the state space of an autonomous dynamical system and is a powerful tool for the analysis and decomposition of nonlinear dynamical systems. In this manuscript, we present a data-driven method for approximating the leading eigenvalues, eigenfunctions, and modes of the Koopman operator. The method requires a data set of snapshot pairs and a dictionary of scalar observables, but does not require explicit governing equations or interaction with a “black box” integrator. We will show that this approach is, in effect, an extension of dynamic mode decomposition (DMD), which has been used to approximate the Koopman eigenvalues and modes. Furthermore, if the data provided to the method are generated by a Markov process instead of a deterministic dynamical system, the algorithm approximates the eigenfunctions of the Kolmogorov backward equation, which could be considered as the “stochastic Koopman operator” (Mezic in Nonlinear Communicated by Oliver Junge. B Matthew O. Williams [email protected] Ioannis G. Kevrekidis [email protected] Clarence W. Rowley [email protected] 1 Program in Applied and Computational Mathematics (PACM), Princeton University, Princeton, NJ 08544, USA 2 Chemical and Biological Engineering Department & PACM, Princeton University, Princeton, NJ 08544, USA 3 Mechanical and Aerospace Engineering Department, Princeton University, Princeton, NJ 08544, USA