def METHOD_NAME(U):
    u = float(U)
    a_0 =-0.57722
    a_1 = 0.99999
    a_2 =-0.24991
    a_3 = 0.05520
    a_4 =-0.00976
    a_5 = 0.00108
    b_1 = 8.57333
    b_2 = 18.05902
    b_3 = 8.63476
    b_4 = 0.26777
    c_1 = 9.57332
    c_2 = 25.63296
    c_3 = 21.09965
    c_4 = 3.95850
    try:
        u < 0
    except KeyError:
        raise RuntimeError("u cannot be negative check input times and radius!")
    if u < 1:
        W = -math.log(u) + a_0 + a_1*u + a_2*u**2 + a_3*u**3 + a_4*u**4 + a_5*u**5
        return W
    if u >= 1:
        W = (math.exp(-u) / u)*((u**4 +b_1*(u**3) + b_2*(u**2) + b_3*u + b_4)/(u**4 + c_1*(u**3) + c_2*(u**2) + c_3*u + c_4))
        return W