Partial directed coherence.

        .. math:: \mathrm{PDC}_{ij}(f) = \\frac{A_{ij}(f)}
                                               {\sqrt{A_{:j}'(f) A_{:j}(f)}}

        References
        ----------
        L. A. Baccalá, K. Sameshima. Partial directed coherence: a new concept
        in neural structure determination. Biol. Cybernetics 84(6): 463-474,
        2001.

def PDC(self):
        """Partial directed coherence.

        .. math:: \mathrm{PDC}_{ij}(f) = \\frac{A_{ij}(f)}
                                               {\sqrt{A_{:j}'(f) A_{:j}(f)}}

        References
        ----------
        L. A. Baccalá, K. Sameshima. Partial directed coherence: a new concept
        in neural structure determination. Biol. Cybernetics 84(6): 463-474,
        2001.
        """
        A = self.A()
        return np.abs(A / np.sqrt(np.sum(A.conj() * A, axis=0, keepdims=True)))