<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)))