Directed transfer function.

        .. math:: \mathrm{DTF}_{ij}(f) = \\frac{H_{ij}(f)}
                                               {\sqrt{H_{i:}(f) H_{i:}'(f)}}

        References
        ----------
        M. J. Kaminski, K. J. Blinowska. A new method of the description of the
        information flow in the brain structures. Biol. Cybernetics 65(3):
        203-210, 1991.

def DTF(self):
        """Directed transfer function.

        .. math:: \mathrm{DTF}_{ij}(f) = \\frac{H_{ij}(f)}
                                               {\sqrt{H_{i:}(f) H_{i:}'(f)}}

        References
        ----------
        M. J. Kaminski, K. J. Blinowska. A new method of the description of the
        information flow in the brain structures. Biol. Cybernetics 65(3):
        203-210, 1991.
        """
        H = self.H()
        return np.abs(H / np.sqrt(np.sum(H * H.conj(), axis=1, keepdims=True)))