'Fourier transform of ARMA polynomial, zero-padded at end to n
The Fourier transform of the ARMA process is calculated as the ratio
of the fft of the MA polynomial divided by the fft of the AR polynomial.
Parameters
n : int
length of array after zero-padding
Returns
fftarma : ndarray
fft of zero-padded arma polynomial'

def fftarma(self, n=None):

    if (n is None):
        n = self.nobs
    return (self.fftma(n) / self.fftar(n))
