data fitting

[Jiri Pachman]

Hi all,
I am trying to determine parameters of equation by fitting it to  
experimental data. The equation is in the form of:

y = a1 * (x*a2)/(1+x^2*a2^2) + a3 * (x*a4)/(1+x^2*a4^2) + ....

a1, a2, a3 etc are the parameters I am trying to find. I started with one  
(a1, a2) and went up to make sure I am not too far with the initial guess.  
I used leasqr and it was ok on theoretical data, but when I try to use it  
on real ones it fails. I get inverse matrix singular to machine precision.  
I found in literature (since this is typical problem from rheology) that  
this kind of problem is ill-posed in its nature and is solved by using  
Thikonov regularization. I just wanted to know, if there is a more robust  
way in octave to fit the data.

thanks

Jiri


--
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Ing. Jiri Pachman
Katedra teorie a technologie vybusnin
Univerzita Pardubice
Studentska 95
532 10 Pardubice

tel.:  +420-46-603-8018
fax.: +420-46-603-8024
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