This document discusses Fourier series and periodic functions. It defines a periodic function as one where f(x+T)=f(x) for some positive number T. The Fourier series of a periodic function with period T can be expressed as the sum of sines and cosines with frequencies that are integer multiples of 1/T. It also discusses even and odd functions, noting that the Fourier series of an even function contains only cosine terms, while the Fourier series of an odd function contains only sine terms. Examples are provided of finding the Fourier series for specific periodic functions.