Model structure simplification of Nonlinear Systems via immersion

This paper concerns the simplification of model structures of nonlinear systems while preserving their input-output maps. The basic technique is an immersion of a system, which is a mapping of the initial state from the original system to another system of an identical input-output map. The necessary and sufficient conditions are presented for the immersibility of a given system into a state-space representation of such particular structures as rational functions or polynomials with respect to the state. The conditions are so mild that many types of nonlinear systems can be represented by rational or polynomial model structures. Moreover, it is also shown that a polynomial state-space representation can always be constructed to be at most quadratic with respect to the state.