A mathematical model is a set of mathematical relations representing the system. These relations may assume many forms, including linear equations, nonlinear equations, integral equations, differential equations, and difference equations. The solution Of these equations represents the system's behavior. Often, this solution is difficult, if not impossible, to find, and certain simplifying assumptions must be made in the mathematical description. Frequently, these approximations and simplifications lead to systems describable by linear ordinary differential equations or difference equations.