def _h_3ab(P):
    """Define the boundary between Region 3a-3b, h=f(P)

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

    Returns
    -------
    h : float
        Specific enthalpy [kJ/kg]

    Examples
    --------
    >>> _h_3ab(25)
    2095.936454
    """
    return 0.201464004206875e4 + 3.74696550136983*P - \
        0.0219921901054187*P**2+0.875131686009950e-4*P**3