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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)))