ABSTRACT
A function containing variables and their derivatives is called a differential expression, and an equation involving differential expressions is called a differential equation. A differential equation is an ordinary differential equation if it contains only one independent variable; it is a partial differential equation if it contains more than one independent variable. In the mathematical texts, the independent variable is generally x, which can be anything such as time, distance, velocity, and pressure. In most of the applications in control systems, the independent variable is time. The convolution method is more laborious compared to the classical method. However, in system analysis, its advantages outweigh the extra work. The development of difference equations is parallel to that of differential equations. In difference equations it is convenient to use operational notation similar to that used in differential equations for the sake of compactness and convenience.
