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TECHNIQUES commonly used at the present time in the analysis and synthesis of feedback-control systems are largely those of constant-linear system theory. In the usual form, these methods stem from a system characterization in terms of ordinary linear differential equations having real constant coefficients. Fortunately, for many purposes this characterization is satisfactory, and in these cases the properties and behavior of the systems can be studied analytically by application of the large body of available linear theory. Frequently, however, practical control systems contain elements, either unavoidably or by design, which are not linear in the engineering sense of being describable by linear differential equations with constant coefficients.