求序列向量xn的傅里叶变换和离散傅里叶变换并画出它们的幅频特性和相频特性曲线，验证离散傅里叶变换是傅里叶变换的等间隔采样，验证IDFT和DFT的唯一性-Find the Fourier transform of the sequence xn vector and the discrete Fourier transform and draw their amplitude-frequency characteristic and phase-frequency characteristic c