Fourier's Transform
Consider the trigonometrical series of the following form \frac{a_0}{2}\sum \limits _{n=1}\infty \left( a_n\cos n\omega x+ b_n\sin n\omega x\right) Since the functions \cos n\omega x\ and \sin n\omega x\ are periodical functions having the period (T=2\pi \omega we say that the series (4.1.1) is a periodical series.