docs: 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)))