Fixing parameters for nonlinear least squares fitting

[Simon Martin <simon dot martin at sheffield dot ac dot uk>]

Hi
I'm working on a program that fits some data using the noninear least 
squares routines from GSL.  I have written a C++ object that implements 
the example from the manual pages.  This works very well.  Now I'd like 
to be able to select which parameters are fitted - experience shows that 
some parameters are best fitted independently of others.  My idea was to 
set the relevent terms in the Jacobian to zero - thinking that this 
would effectively meant that these parameters were at their ideal 
values.  Unfortunately, doesn't work.  The fixed parameter goes to 
infinity and the other parameters go to NaN.

Does anybody know A way around this?

Simon