def par_u1step(i):
    r"""Dual variable update for :math:`\mathbf{u}_{1,G_i}`, one of the
    disjoint problems for updating :math:`\mathbf{u}_1`.

    Parameters
    ----------
    i : int
      Index of grouping to update
    """

    global mp_U1
    grpind = slice(mp_grp[i], mp_grp[i+1])
    mp_U1[grpind] += mp_alpha*(mp_X[grpind] - mp_Y1[grpind])