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Groups > comp.graphics.apps.gnuplot > #2180

Fit with "multi-part" function?

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Hi,
I have often successfully used Gnuplot's excellent fitting function, so I'm a bit perplexed by the problem I encounter now. Consider this:

b2=1./(3.*delta0-1)
fnaux(n,bn,x,y)=1.+(n/(3.*bn*KT0))*(x-rbsm*alpha0*KT0*(y-T0))
fn(n,bn,x,y)=1./(1.-bn+bn*(fnaux(n,bn,x,y))**(1./n))**3
Vth(x,y)=(V0/rbsm)*(fn(2,b2,x,y)-fn(2,b2,x,T0))

T0, KT0, and V0 are fixed constants, and what I try to do is:

fit Vth(x,y) 'pV_db.dat' u 2:1:3 via alpha0,delta0,rbsm

This results in a singular matrix, even though the initial guesses for alpha0,delta0,rbsm are close to the expected solution.
My question really is this: can I not use auxiliary functions like b2, fnaux, fn for writing Vth when performing the fit? Or, in other words, will something like b2 remain fixed with the initial guess, even when the fitting procedure is supposed to vary delta0, on which b2 depends? This would obviously limit the complexity of function models I can use for fitting data with Gnuplot.

Thomas

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Fit with "multi-part" function? trg818@gmail.com - 2013-10-29 16:57 -0700
  Re: Fit with "multi-part" function? Karl <mail.kfr@gmx.net> - 2013-10-30 10:43 +0100
    Re: Fit with "multi-part" function? trg818@gmail.com - 2013-10-30 06:44 -0700
      Re: Fit with "multi-part" function? Karl <mail.kfr@gmx.net> - 2013-10-30 16:18 +0100

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