def _Backward3a_v_Ph(P, h):
    """Backward equation for region 3a, v=f(P,h)

    Parameters
    ----------
    P : float
        Pressure [MPa]
    h : float
        Specific enthalpy [kJ/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 4

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

    Examples
    --------
    >>> _Backward3a_v_Ph(20,1700)
    0.001749903962
    >>> _Backward3a_v_Ph(100,2100)
    0.001676229776
    """
    I = [-12, -12, -12, -12, -10, -10, -10, -8, -8, -6, -6, -6, -4, -4, -3, -2,
         -2, -1, -1, -1, -1, 0, 0, 1, 1, 1, 2, 2, 3, 4, 5, 8]
    J = [6, 8, 12, 18, 4, 7, 10, 5, 12, 3, 4, 22, 2, 3, 7, 3, 16, 0, 1, 2, 3,
         0, 1, 0, 1, 2, 0, 2, 0, 2, 2, 2]
    n = [0.529944062966028e-2, -0.170099690234461, 0.111323814312927e2,
         -0.217898123145125e4, -0.506061827980875e-3, 0.556495239685324,
         -0.943672726094016e1, -0.297856807561527, 0.939353943717186e2,
         0.192944939465981e-1, 0.421740664704763, -0.368914126282330e7,
         -0.737566847600639e-2, -0.354753242424366, -0.199768169338727e1,
         0.115456297059049e1, 0.568366875815960e4, 0.808169540124668e-2,
         0.172416341519307, 0.104270175292927e1, -0.297691372792847,
         0.560394465163593, 0.275234661176914, -0.148347894866012,
         -0.651142513478515e-1, -0.292468715386302e1, 0.664876096952665e-1,
         0.352335014263844e1, -0.146340792313332e-1, -0.224503486668184e1,
         0.110533464706142e1, -0.408757344495612e-1]

    Pr = P/100
    nu = h/2100
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (Pr+0.128)**i * (nu-0.727)**j
    return 0.0028*suma