def _hbc_P(P):
    """Define the boundary between Region 2b and 2c, h=f(P)

    >>> "%.6f" % _hbc_P(100)
    '3516.004323'
    """
    return 0.26526571908428e4+((P-0.45257578905948e1)/1.2809002730136e-4)**0.5