def _Backward2a_T_Ps(P, s):
    """Backward equation for region 2a, T=f(P,s)

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

    Returns
    -------
    T : float
        Temperature, [K]

    References
    ----------
    IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the
    Thermodynamic Properties of Water and Steam August 2007,
    http://www.iapws.org/relguide/IF97-Rev.html, Eq 25

    Examples
    --------
    >>> _Backward2a_T_Ps(0.1,7.5)
    399.517097
    >>> _Backward2a_T_Ps(2.5,8)
    1039.84917
    """
    I = [-1.5, -1.5, -1.5, -1.5, -1.5, -1.5, -1.25, -1.25, -1.25, -1.0, -
        1.0, -1.0, -1.0, -1.0, -1.0, -0.75, -0.75, -0.5, -0.5, -0.5, -0.5, 
        -0.25, -0.25, -0.25, -0.25, 0.25, 0.25, 0.25, 0.25, 0.5, 0.5, 0.5, 
        0.5, 0.5, 0.5, 0.5, 0.75, 0.75, 0.75, 0.75, 1.0, 1.0, 1.25, 1.25, 
        1.5, 1.5]
    J = [-24, -23, -19, -13, -11, -10, -19, -15, -6, -26, -21, -17, -16, -9,
        -8, -15, -14, -26, -13, -9, -7, -27, -25, -11, -6, 1, 4, 8, 11, 0, 
        1, 5, 6, 10, 14, 16, 0, 4, 9, 17, 7, 18, 3, 15, 5, 18]
    n = [-392359.83861984, 515265.7382727, 40482.443161048, -
        321.93790923902, 96.961424218694, -22.867846371773, -
        449429.14124357, -5011.8336020166, 0.35684463560015, 44235.33584819,
        -13673.388811708, 421632.60207864, 22516.925837475, 474.42144865646,
        -149.31130797647, -197811.26320452, -23554.39947076, -
        19070.616302076, 55375.669883164, 3829.3691437363, -603.91860580567,
        1936.3102620331, 4266.064369861, -5978.0638872718, -704.01463926862,
        338.36784107553, 20.862786635187, 0.033834172656196, -
        4.3124428414893e-05, 166.53791356412, -139.86292055898, -
        0.78849547999872, 0.072132411753872, -0.0059754839398283, -
        1.2141358953904e-05, 2.3227096733871e-07, -10.538463566194, 
        2.0718925496502, -0.072193155260427, 2.074988708112e-07, -
        0.018340657911379, 2.9036272348696e-07, 0.21037527893619, 
        0.00025681239729999, -0.012799002933781, -8.2198102652018e-06]
    Pr = P / 1
    sigma = s / 2
    T = 0
    for i, j, ni in zip(I, J, n):
        T += ni * Pr ** i * (sigma - 2) ** j
    return T
