离散傅里叶变换-DFT（FFT基础）
https://blog.csdn.net/zhangxz259/article/details/81627341
https://blog.csdn.net/qq295456059/article/details/50354222
https://zhuanlan.zhihu.com/p/282914222 除以N的文章
https://xiong535.blog.csdn.net/article/details/105125387
https://www.zhihu.com/question/21314374/answer/542909849 tan