docs: 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)))