def _Backward3a_v_Ps(P, s):
    """Backward equation for region 3a, v=f(P,s)

    Parameters
    ----------
    P : float
        Pressure [MPa]
    s : float
        Specific entropy [kJ/kgK]

    Returns
    -------
    v : float
        Specific volume [m³/kg]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for the
    Functions T(p,h), v(p,h) and T(p,s), v(p,s) for Region 3 of the IAPWS
    Industrial Formulation 1997 for the Thermodynamic Properties of Water and
    Steam, http://www.iapws.org/relguide/Supp-Tv%28ph,ps%293-2014.pdf, Eq 8

    Examples
    --------
    >>> _Backward3a_v_Ps(20,3.8)
    0.001733791463
    >>> _Backward3a_v_Ps(100,4)
    0.001555893131
    """
    I = [-12, -12, -12, -10, -10, -10, -10, -8, -8, -8, -8, -6, -5, -4, -3, -3,
         -2, -2, -1, -1, 0, 0, 0, 1, 2, 4, 5, 6]
    J = [10, 12, 14, 4, 8, 10, 20, 5, 6, 14, 16, 28, 1, 5, 2, 4, 3, 8, 1, 2, 0,
         1, 3, 0, 0, 2, 2, 0]
    n = [0.795544074093975e2, -0.238261242984590e4, 0.176813100617787e5,
         -0.110524727080379e-2, -0.153213833655326e2, 0.297544599376982e3,
         -0.350315206871242e8, 0.277513761062119, -0.523964271036888,
         -0.148011182995403e6, 0.160014899374266e7, 0.170802322663427e13,
         0.246866996006494e-3, 0.165326084797980e1, -0.118008384666987,
         0.253798642355900e1, 0.965127704669424, -0.282172420532826e2,
         0.203224612353823, 0.110648186063513e1, 0.526127948451280,
         0.277000018736321, 0.108153340501132e1, -0.744127885357893e-1,
         0.164094443541384e-1, -0.680468275301065e-1, 0.257988576101640e-1,
         -0.145749861944416e-3]

    Pr = P/100
    sigma = s/4.4
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (Pr+0.187)**i * (sigma-0.755)**j
    return 0.0028*suma