def y0astep():
    r"""The serial component of the step to minimise the augmented
    Lagrangian with respect to :math:`\mathbf{y}_0`.
    """

    global mp_b
    mp_b[:] = mp_inv_off_diag * np.sum((mp_S + mp_rho*(mp_DX+mp_U0)),
                                       axis=mp_axisM, keepdims=True)