Mathematics of the DFT: https://ccrma.stanford.edu/~jos/mdft/
Jupyter notebooks: https://github.com/khiner/notebooks/tree/master/mathematics_of_the_dft
https://jakevdp.github.io/blog/2013/08/28/understanding-the-fft/
http://www.cbcity.de/die-fft-mit-python-einfach-erklaert
https://jakevdp.github.io/blog/2015/02/24/optimizing-python-with-numpy-and-numba/