def cnst_A0T(self, Y0):
        r"""Compute :math:`A_0^T \mathbf{y}_0` component of
        :math:`A^T \mathbf{y}` (see :meth:`.ADMMTwoBlockCnstrnt.cnst_AT`).
        """

        # This calculation involves non-negligible computational cost. It
        # should be possible to disable relevant diagnostic information
        # (dual residual) to avoid this cost.
        Y0f = sl.rfftn(Y0, None, self.cri.axisN)
        if self.cri.Cd == 1:
            return sl.irfftn(np.conj(self.Df) * Y0f, self.cri.Nv,
                             self.cri.axisN)
        else:
            return sl.irfftn(sl.inner(
                np.conj(self.Df), Y0f, axis=self.cri.axisC),
                self.cri.Nv, self.cri.axisN)