[Help-gsl] GSL 1.9: Levenberg-Marquardt non-linear fit with selected invariant coefficients not to be fit, HOWTO?

[O. Hartmann]

Hello,

while using GSL the first time (coming from some Numerical Recipes in C 
code pieces) I ran into trouble solving a non-linear least square problem.
The problem ist simple, not to say trivial, some points need to get 
fitted in the usual way. The modell-function is a polynomial expression 
of degree 15 with 16 coefficients (a_0 - a_15) and in most cases we only 
fit the first coefficient, the constant (a_0). Sometimes we need to 
select some other coefficients to be fitted for making the polynomial 
fitting the dataset.
Well, The Numerical Recipes code offers an elegant method using a 'mask' 
for each coefficient, if the corresponding switch in the mask (the mask 
represents the amount of coefficients) is set to 'false' or 0, the 
coefficient does not get touched and fitted, if set to else, the 
coefficient is handled as usual and gets fitted. These operations are 
done while preparing the covariant matrix.
The GSL routines, as shown in the example in chapter 37: Nonlinear 
Least-Square Fitting seems not to offer a simple 'switch' to do the same 
as mentioned above. Does anyone do have a hint?
One idea was to test if an coefficient has been marked 'fittable' while 
setting up the Jacobian matrix, but this is stupid and my math isn't 
that descent developed ...

Thanks in advance,
Oliver