Realizability Condition for Nonlinear Feedback Systems

Mathematical models of nonlinear systems frequently lead to impossible solutions that can never be realized physically. In this paper the requirements of a realizable model are explored. First, a definition is proposed for the phrase "sufficient condition for system realizability;" the definition requires the preservation of approximations under interconnections. Second, a realizability condition is provided which is a generalization-for nonlinear systems-of the concepts of delay and attenuation of high frequencies. The condition is stated in terms of observable input-output behavior.