def obfn_f(self):
        r"""Compute :math:`f(\mathbf{x}) = \sum_i f(\mathbf{x}_i)`
        component of ADMM objective function.
        """

        obf = 0.0
        for i in range(self.Nb):
            obf += self.obfn_fi(self.obfn_fvar(i), i)
        return obf