Nonlinear Least Squares Fitting

[Andreas Gros]

Hello,

I'd like to submit a "bug" in the solver:
gsl_multifit_fdfsolver_lmsder.

In the section "Providing the function to be minimized" where the
function gsl_multifit_function_fdf is described, you mention the
function f that should store the distance between the values
calculated by the algorithm and the provided values. 
As I tested the algorithm I've found, that this distance is not taken
as a "square", instead the algebraic sign of the distance seems to be
of some importance. My results were quite different, whenever I took
(y[i]-Yi) instead of (Yi-y[i]) (as it is shown in the example).

Taking the latter the solver works fine, taking the first distance the
results have been quite funny.

Andreas Gros 


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