def TB23_hs(h, s):
    """function TB23_hs = TB23_hs(h, s)

    Section 4.3 Region boundary 1to3  & 3to2 as a functions of s

    Supplementary Release on Backward Equations () , p h s for Region 3, Chapter 4.6 page 25.
    """
    Ii = [-12, -10, -8, -4, -3, -2, -2, -2, -2, 0, 1, 1, 1, 3, 3, 5, 6, 6, 
        8, 8, 8, 12, 12, 14, 14]
    Ji = [10, 8, 3, 4, 3, -6, 2, 3, 4, 0, -3, -2, 10, -2, -1, -5, -6, -3, -
        8, -2, -1, -12, -1, -12, 1]
    ni = [0.00062909626082981, -0.000823453502583165, 5.15446951519474e-08,
        -1.17565945784945, 3.48519684726192, -5.07837382408313e-12, -
        2.84637670005479, -2.36092263939673, 6.01492324973779, 
        1.48039650824546, 0.000360075182221907, -0.0126700045009952, -
        1221843.32521413, 0.149276502463272, 0.698733471798484, -
        0.0252207040114321, 0.0147151930985213, -1.08618917681849, -
        0.000936875039816322, 81.9877897570217, -182.041861521835, 
        2.61907376402688e-06, -29162.6417025961, 1.40660774926165e-05, 
        7832370.62349385]
    Sigma = s / 5.3
    eta = h / 3000
    teta = 0
    for i in range(0, 25):
        teta = teta + ni[i] * (eta - 0.727) ** Ii[i] * (Sigma - 0.864) ** Ji[i]
    return teta * 900
