Determining the response of nonlinear systems to arbitrary inputs

IF AN ENGINEER is presented with a general linear system, a ¿black box,¿ let us say, having an input and an output, the first question he will usually ask is: What is (or should be) the relationship between its input and output? This is a question which can be answered in many ways. One can determine or prescribe the response of the box to impulses or step functions or sine waves, and from these calculate the response to any input, regardless of its character. The concept here is essentially that of impedance, a term which may be defined in a variety of ways, but the central idea of all the definitions is that, in some fashion, the general relationship between the stimulus and the response of a system is specified.