And still further: the function can be piecewise decomposed into separate polynomials, which is what the pervasive SunPro code in everyone's libc does. There's

And still further: the function can be piecewise decomposed into separate polynomials, which is what the pervasive SunPro code in everyone's libc does. There's one "mostly parabolic" function to handle the curvy top, and another "mostly linear" one to handle the zero crossing region.