text: Charecteristics of physical systems is defined by differential equation. For example  a second order linear differential equation can define an RLC circuit. An antennae and it's polar diagram can be defined by a Legendre differential equation from which the whole antennae can be described and designed. A Bessel differential equation can define movement of planets in space. This can also be used to define cetain type of Electrical cirucuits. Butterworth, Chebyshev etc; polynomials define certain other type of Electrical circuits. So basically these are mathematical models of physical systems.