Re: Minuit problem

Could yous send the shortest possible (but running) example showing this problem?

Rene Brun

andreyk1_at_post.tau.ac.il wrote:
> Dear Roottalk,
>
> I use a chi square minimization routin minuit. I call it in the following way:
>
> ------------------------------------------------------------------
>
> TGraph2DErrors *dte = new TGraph2DErrors(data_length);
>
> for(int i=0;i<file_length;i++){
> dte->SetPoint(i,x_b[i],Q2[i],F2[i]);
> dte->SetPointError(i,er_x[i],er_y[i],er_z[i]);
> };
>
> TF2 *func = new TF2("fit", fitted_function, 0,0.013, 0,150, 3);
> func->SetParName(0,"mu0");
> func->SetParName(1,"A");
> func->SetParName(2,"Omega_0");
>
> func->SetParameters(param);
> dte->Fit("fit");
> ------------------------------------------------------------------
>
> where "fitted_function" is my function with three parameters and two variables
> (x_b,Q2)
>
> The problem is: the result of minuit or a value of chi square depends on the
> initial values of my parameters, i.e. a little change in the initial values
> affects considerably on the result of minimization (a value of chi squre). I
> think that the reason for this is that my function has many local minima and one
> global minimum, but I can not achieve it.
>
> Can I do something with it?
>
>
>
>
Received on Mon Apr 23 2007 - 08:20:44 CEST