A representation theorem for ground stable nonlinear systems with piecewise-constant inputs is presented. It is a three-dimensional system of differential equations augmented by an algebraic output equation. The representation is characterized by

parameters, where

is the number of harmonics used to represent transients. Five of the parameters are positive constants, and the remaining

parameters are functions of either

or

variables. Applications involving the macromodeling of higher dimensional linear and nonlinear systems are presented.