The Fourier series is the basis for much of digital signal processing, ... There are no equations, just engaging animations that show how any function can be decomposed into a set of sine waves.

The Theory of Functions of a Real Variable and the Theory of Fourier's Series. By Prof. E. W. Hobson. Vol. 1. Third edition, revised throughout and enlarged.

Reviews ordinary differential equations, including solutions by Fourier series. Physical derivation of the classical linear partial differential equations (heat, wave, and Laplace equations). Solution ...