https://en.wikipedia.org/wiki/File:Fourier_series_square_wave_circles_animation.gif
https://en.wikipedia.org/wiki/File:Fourier_series_sawtooth_wave_circles_animation.gif
https://zhuanlan.zhihu.com/p/19759362?utm_source=wechat_session&utm_medium=social&from=singlemessage
