[ROOT] Histogram fitting with landau

From: Bob Schaefer (bob@xcom.gsfc.nasa.gov)
Date: Tue Sep 18 2001 - 19:24:23 MEST


Hello,

  I seemed to find 2 problems while using root to fit histograms. 1) Funny 
behavior in fitting sums of functions when a landau function is involved, and 
2) Getting the SetOptFit to display the parameters as described.

  
1)  I tried fitting a sum ("total") of a landau function and an exponential 

    TH1F *hist;

    // fitting functions
    TF1 *exp1, *land, *total;
    // fitting parameters
    Double_t par[5];    
    
    land = new TF1("land", "landau(0)",20,800);
    hist->Fit(land,"WQ0","",300,800);
    hist->Fit(exp1,"WQ0+","",20,200);

    exp1 = new TF1("exp1","expo(0)", 20, 800);

    land->GetParameters(&par[0]);
    exp1->GetParameters(&par[3]);

    total->SetParameters(&par[0]);

    total = new TF1("total","expo(3)+landau(0)", 20, 800);
    hist->Fit(total,"WR");

This works fine unless I try to do the fitting with the exponential parameters 
FIRST in the parameter list, i.e., 

    land->GetParameters(&par[2]);
    exp1->GetParameters(&par[0]);

    total->SetParameters(&par[0]);
    total = new TF1("total","expo(0)+landau(2)", 20, 800);
    hist->Fit(total,"WR");
 
In which case the fit fails, complaining that the ERROR MATRIX IS NOT POSITIVE 
DEFINITE.  


2) Fit parameters not being displayed 

    According to the documentation: 
    
    gStyle->SetOptFit(0101);
    
    should print the names/values of the parameters AND the Chi-squared.  It 
does not print the chi-squared, although it does print the parameters. 

    However, if I use 

    gStyle->SetOptFit(10101);
    
    it prints both chi sq and the parameters?!
    
Thanks,
  Bob Schaefer

    
    
    

Root 3.1/02 on LinuxPPC2000 running on a Mac G4 dual processor.
   

----------------------------------------------------------
Robert K. Schaefer       Email: bob@milkyway.gsfc.nasa.gov
NASA/GSFC Code 664       Phone: 301-286-0878
Greenbelt, MD 20771      FAX:   301-286-1629
----------------------------------------------------------



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