Harmonic spectrum

A harmonic spectrum is a spectrum containing only frequency components whose frequencies are whole number multiples of the fundamental frequency; such frequencies are known as harmonics. "The individual partials are not heard separately but are blended together by the ear into a single tone."
In other words, if formula_1 is the fundamental frequency, then a harmonic spectrum has the form 
A standard result of Fourier analysis is that a function has a harmonic spectrum if and only if it is periodic.