def obfn_dfd(self):
        r"""Compute data fidelity term :math:`(1/2) \| D \mathbf{x} -
        \mathbf{s} \|_2^2`.
        """

        return 0.5*np.linalg.norm((self.D.dot(self.obfn_fvar()) - self.S))**2
r"""Compute data fidelity term :math:`(1/2) \| D \mathbf{x} -
        \mathbf{s} \|_2^2`.