def _tef_P(P):
    """Define the boundary between Region 3e-3f, T=f(P)

    Parameters
    ----------
    P : float
        Pressure, [MPa]

    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. 3

    Examples
    --------
    >>> _tef_P(40)
    713.9593992
    """
    return 3.727888004 * (P - 22.064) + 647.096
