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FFT Discrete Fourier transform.
    FFT(X) is the discrete Fourier transform (DFT) of vector X.  For
    matrices, the FFT operation is applied to each column. For N-D
    arrays, the FFT operation operates on the first non-singleton
    dimension.
 
    FFT(X,N) is the N-point FFT, padded with zeros if X has less
    than N points and truncated if it has more.
 
    FFT(X,[],DIM) or FFT(X,N,DIM) applies the FFT operation across the
    dimension DIM.
    
    For length N input vector x, the DFT is a length N vector X,
    with elements
                     N
       X(k) =       sum  x(n)*exp(-j*2*pi*(k-1)*(n-1)/N), 1 <= k <= N.
                    n=1
    The inverse DFT (computed by IFFT) is given by
                     N
       x(n) = (1/N) sum  X(k)*exp( j*2*pi*(k-1)*(n-1)/N), 1 <= n <= N.
                    k=1
 
    The relationship between the DFT and the Fourier coefficients a and b in
                N/2
    x(n) = a0 + sum a(k)*cos(2*pi*k*t(n)/(N*dt))+b(k)*sin(2*pi*k*t(n)/(N*dt))
                k=1
    is
       a0 = X(1)/N, a(k) = 2*real(X(k+1))/N, b(k) = -2*imag(X(k+1))/N,
    where x is a length N discrete signal sampled at times t with spacing dt.