Non linear parameter estimation

[a.mlw.walker]

Hi I have a nonlinear expression I would like to estimate the three parameters of, however I require a computer to do it. I would like a least squares optimization method or some form that required as little as possible from the user as the partial derivatives are quite nasty. I have considered the levenberg marquardt method however documentation on using it is quite slim.

I wondered whether anyone could suggest another method (with better documentation) and possibly know of existing computer example of using it. I understand matlab and C. However I really require to be able to use the algorithm in C.

Thanks