Little example of applying Fourier series to circuits. By expressing the input signal as the sum of a few sine waves, the circuit can be very easily analysed regardless of how complicated the input signal is, and you avoid having to use differential equations to do so.
On the left is a square wave source and its RC response. On the right is the Fourier representation of both traces in the original circuit. Only the first 3 harmonics are shown, but the more you add, the more it resembles the original traces.
