What technique shows that a square wave is made up of a sine wave and its odd harmonics?

Fourier analysis is the mathematical process of decomposing a complex periodic waveform into a series of fundamental sine and cosine waves. This is the underlying principle behind spectrum analyzers and digital signal processing. When applied to a perfect square wave, Fourier analysis proves that the wave is entirely composed of a fundamental sine wave added together with an infinite number of its odd harmonics (3rd, 5th, 7th, etc.), each at progressively smaller amplitudes. The other choices are incorrect because vector analysis is used to determine phase and magnitude relationships (like in impedance), and differential analysis deals with rates of change, neither of which identify harmonic content.