def hB13_s(s):
    """function hB13_s = hB13_s(s)

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

    Supplementary Release on Backward Equations ( ) , p h s for Region 3, Chapter 4.5 page 23.
    """
    Ii = [0, 1, 1, 3, 5, 6]
    Ji = [0, -2, 2, -12, -4, -3]
    ni = [0.913965547600543, -4.30944856041991e-05, 60.3235694765419, 
        1.17518273082168e-18, 0.220000904781292, -69.0815545851641]
    Sigma = s / 3.8
    eta = 0
    for i in range(0, 6):
        eta = eta + ni[i] * (Sigma - 0.884) ** Ii[i] * (Sigma - 0.864) ** Ji[i]
    return eta * 1700
