A Dual Koopman Approach to Observer Design for Nonlinear Systems

The Koopman operator approach to the state estimation problem for nonlinear systems is a promising research area. The main goal of this paper is an attempt to provide a rigorous theoretical framework for this approach. In particular, the (linear) dual Koopman system is introduced and studied in an infinite dimensional context. Moreover, new concepts of observability and detectability are defined in the dual Koopman system, which are shown to be equivalent to the observability and detectability of the nonlinear system, respectively. The theoretical framework is applied to a class of holomorphic dynamics. For this class, a Luenberger-type observer is designed for the dual Koopman system via a spectral method, yielding an estimate of the state of the nonlinear system. A particular attention is given to the existence of an appropriate solution to the dual Koopman system and observer, which are defined in the Hardy space on the polydisc. Spectral observability and detectability conditions are derived in this setting, and the exponential convergence of the Koopman observer is shown. Finally, numerical experiments support the theoretical findings.