Ken, If you just need a quick answer to your problem, I would like you to try my tool called wasora, which amongst other things, is a high-level interface to some of the GSL tools.
I have added your fitting problem to the examples subdirectory (I made up the data). You can take a look at it by first downloading the tarball from http://www.talador.com.ar/jeremy/wasora/ and the looking at examples/fit.was Sadly, the code is lacking good documentation. So far all I have are examples of usage. In the following link http://www.talador.com.ar/jeremy/wasora/realbook/._realbook015.html some comparison between GSL examples written in C as taken from the manual and the (almost) the same results obtained with wasora by using a far more simple high-level input file. Regarding your fitting problem, wasora takes care of numerically computing the gradient of the function with respect to the parameters if explicit algebraic expressions are not given. -- jeremy On Thursday 03 July 2014 18:07:40 k...@kenobrien.org wrote: > Hi, > I am trying to fit the nonlinear equation: > P = C0 + C1*X + C2*X^r > where C0, C1, C2 and r are variables to be fitted, P and X are vectors > of known observations. > I have read the docs and I'm trying to modify the example program. I'm > struggling to provide the expb_f, expb_df, expb_fdf functions. > I would like advice on how to proceed. > Regards, > Ken