fft
fft: 複素フーリエ変換.周波数は0, 1/L, 2/L, ... 0.5, ....2/L, 1/L. ここでLはデータの数．
ifft: 逆複素フーリエ変換
fft: complex Fourier transform at frequencies of 0, 1/L, 2/L, ..., 0.5, 2/L, 1/L. Here L is the number of data point.
ifft: inverse complex Fourier transform.
fft: 複素フーリエ変換.周波数は0, 1/L, 2/L, ... 0.5, ....2/L, 1/L. ここでLはデータの数．
ifft: 逆複素フーリエ変換
fft: complex Fourier transform at frequencies of 0, 1/L, 2/L, ..., 0.5, 2/L, 1/L. Here L is the number of data point.
ifft: inverse complex Fourier transform.