[Help-gsl] Nonlinear fitting

[ken]

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