Hi,

I am trying to duplicate the results of the tutorial here: root.cern.ch/root/html/tutorials … als.C.html.

The tutorial runs fine, however I cannot seem to duplicate the results for my custom function fit. My code looks like:

```
TFile * n0 = new TFile("/home/euix/Desktop/clumeq_backup/backup_oct30/prunstuff/pythia_v95_combined_ht_TeV_exclusive_projection.root");
TH2D * jetpt_N = (TH2D*)gDirectory->Get("jetpt_N");
jetpt_N->ProjectionX("jet",4,4);
TCanvas *myc = new TCanvas("myc","Confidence intervals on the fitted function",1200, 500);
myc->SetLogy();
TF1 * dijet_fit=new TF1("dijet_fit"," ([0]*(1-x/8)^[1] )/(x/8)^([2])",1.6,3.0);
dijet_fit->SetParameters(100,12,3);
jet->Draw();
TFitResultPtr fit_results;
fit_results=jet->Fit("dijet_fit","VSR0");
double * points[100];
double * results[100];
TH1D * hint = new TH1D("hint","hint",100,0,10);
(TVirtualFitter::GetFitter())->GetConfidenceIntervals(hint)
hint->SetStats(kFALSE);
hint->SetFillColor(2);
hint->Draw("e3 same");
```

When I plot “hint” I see there is nothing in the histogram. I don’t understand what is going on, the fit seems to return good parameters:

```
MIGRAD MINIMIZATION HAS CONVERGED.
MIGRAD WILL VERIFY CONVERGENCE AND ERROR MATRIX.
START COVARIANCE MATRIX CALCULATION.
EIGENVALUES OF SECOND-DERIVATIVE MATRIX:
5.2906e-05 6.0746e-02 2.9392e+00
COVARIANCE MATRIX CALCULATED SUCCESSFULLY
FCN=19.6143 FROM MIGRAD STATUS=CONVERGED 581 CALLS 582 TOTAL
EDM=4.07242e-08 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 p0 1.59604e+02 1.86704e+01 3.73231e-04 -5.07177e-04
2 p1 1.01555e+01 1.40804e-01 6.99342e-06 4.11422e-02
3 p2 5.39169e+00 5.47148e-02 2.57096e-06 -6.47370e-02
EXTERNAL ERROR MATRIX. NDIM= 25 NPAR= 3 ERR DEF=1
3.486e+02 2.623e+00 -1.021e+00
2.623e+00 1.983e-02 -7.660e-03
-1.021e+00 -7.660e-03 2.994e-03
PARAMETER CORRELATION COEFFICIENTS
NO. GLOBAL 1 2 3
1 0.99996 1.000 0.998 -0.999
2 0.99975 0.998 1.000 -0.994
3 0.99989 -0.999 -0.994 1.000
Info in <Minimize>: Finished to run MIGRAD - status 0
EXTERNAL ERROR MATRIX. NDIM= 3 NPAR= 3 ERR DEF=1
3.486e+02 2.623e+00 -1.021e+00
2.623e+00 1.983e-02 -7.660e-03
-1.021e+00 -7.660e-03 2.994e-03
FCN=19.6143 FROM MIGRAD STATUS=CONVERGED 581 CALLS 582 TOTAL
EDM=4.07242e-08 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER PARABOLIC MINOS ERRORS
NO. NAME VALUE ERROR NEGATIVE POSITIVE
1 p0 1.59604e+02 1.86704e+01
2 p1 1.01555e+01 1.40804e-01
3 p2 5.39169e+00 5.47148e-02
```

Can anyone advise?

Thank You,

Kuhan