multifit_nlin on linear data

[Brian Gough]

tlb@tlb.org writes:
 > I have a multiparameter fitting problem which is nonlinear in general,
 > but has linear behavior in some regions where I can exactly calculate
 > the derivatives. So sometimes when I use gsl_multifit_fdfsolver it's
 > able to jump to the correct solution in a single jump, as you'd expect
 > it to.
 > 
 > Should I be using a different termination critereon than
 > gsl_multifit_test_delta?

I'd recommend using a different termination criterion.  The step size
is not a reliable indicator on its own.  ETOLF indicates that the
algorithm can't go any further downhill, so you can usually
take that as signalling a minimum if you know that the behavior
of the function is reasonable.

regards
Brian Gough