离散傅里叶变换(DFT) 定义 X(k)=∑n=0N−1x(n)e−j2π/N⋅kn=∑n=0N−1x(n)WNknX(k)=\sum_{n=0}^{N-1}x(n)e^{-j2\pi/N\cdot kn}=\sum_{n=0}^{N-1}x(n)W_N^{kn}X(k)=