def _txx_P(P, xy):
    """Define the boundary between 3x-3y, T=f(P)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    xy: string
        Subregions options: cd, gh, ij, jk, mn, qu, rx, uv

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

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for Specific
    Volume as a Function of Pressure and Temperature v(p,T) for Region 3 of the
    IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water
    and Steam, http://www.iapws.org/relguide/Supp-VPT3-2016.pdf, Eq. 1

    Examples
    --------
    >>> _txx_P(25,"cd")
    649.3659208
    >>> _txx_P(23,"gh")
    649.8873759
    >>> _txx_P(23,"ij")
    651.5778091
    >>> _txx_P(23,"jk")
    655.8338344
    >>> _txx_P(22.8,"mn")
    649.6054133
    >>> _txx_P(22,"qu")
    645.6355027
    >>> _txx_P(22,"rx")
    648.2622754
    >>> _txx_P(22.3,"uv")
    647.7996121
    """
    ng = {'cd': [585.276966696349, 2.78233532206915, -0.0127283549295878, 
        0.000159090746562729], 'gh': [-24928.4240900418, 4281.43584791546, 
        -269.02917314013, 7.51608051114157, -0.0787105249910383], 'ij': [
        584.814781649163, -0.616179320924617, 0.260763050899562, -
        0.00587071076864459, 5.15308185433082e-05], 'jk': [617.229772068439,
        -7.70600270141675, 0.697072596851896, -0.0157391839848015, 
        0.000137897492684194], 'mn': [535.339483742384, 7.61978122720128, -
        0.158365725441648, 0.00192871054508108], 'qu': [565.603648239126, 
        5.29062258221222, -0.102020639611016, 0.00122240301070145], 'rx': [
        584.561202520006, -1.02961025163669, 0.243293362700452, -
        0.00294905044740799], 'uv': [528.199646263062, 8.90579602135307, -
        0.222814134903755, 0.00286791682263697]}
    n = ng[xy]
    Pr = P / 1
    T = 0
    for i, ni in enumerate(n):
        T += ni * Pr ** i
    return T
