Double_t fitf(Double_t *x, Double_t *par)
{  
  Double_t fitval =(par[0]*x[1]+par[1]*x[1]*x[1])*
   (TMath::Exp(par[2]*x[0]*x[0]+par[3]*x[0]+par[4]));
  return fitval;
}

void lemaire1()
{
  const Int_t    n    = 34;
  const Int_t    npar =  5;
  const Double_t zero = 0.0;
  Int_t ixy,i;
  Double_t aa;
  Double_t res;
  Double_t ressum;
  Double_t chi2;
  Double_t param[npar];
  Double_t ezfit[n];
  Double_t zzfit[n];

  Double_t x[n] = { 1500.0, 1700.0, 1900.0, 2100.0, 2300.0, 2500.0, 2700.0,
                    1500.0, 1700.0, 1900.0, 2500.0, 3000.0, 3500.0, 1500.0,
                    1900.0, 2500.0, 3000.0, 3500.0, 4000.0, 5000.0, 1500.0,
                    1900.0, 2500.0, 3000.0, 3500.0, 4000.0, 4500.0, 5000.0,
                    1500.0, 1900.0, 2500.0, 3000.0, 4000.0, 5000.0 };
  Double_t y[n] = { 0.01,  0.01,  0.01,  0.01,  0.01,  0.01, 0.01,
                    0.02,  0.02,  0.02,  0.02,  0.02,  0.02, 0.05,
                    0.05,  0.05,  0.05,  0.05,  0.05,  0.05, 0.07,
                    0.07,  0.07,  0.07,  0.07,  0.07,  0.07, 0.07,
                    0.1 ,  0.1,   0.1 ,  0.1,   0.1 ,  0.1 };
  Double_t z[n] = {  136.5,  67.75,  35.97,   19.65,  10.98,  6.56,   3.94,
                     537.2, 271.0 , 139.7 ,   24.74,   7.3 ,  2.4, 3187.7 ,
                     819.0, 149.0 ,  43.3 ,   14.0,    4.52,  0.4, 5953.2,
                    1574.0, 276.0 ,  79.9 ,   26.2,    7.8 ,  2.73,  0.93,
                   12476.4,3157.5,  577.36,  163.8,   17.6 ,  2.11 };

  TH2D *h2 = new TH2D("h2","signal 2d",60,0.,6000.,20,0.,0.2);
  h2->FillN(n,x,y,z,1);
  UInt_t canv_w=900;
  UInt_t canv_h=636;
  gStyle->SetOptLogx(0);
  gStyle->SetOptLogy(0);
  gStyle->SetOptLogz(1); 
  gStyle->SetOptFit(0001);
  TCanvas *MyC1 = new TCanvas("MyC1","Graph ", canv_w ,canv_h);
  TF2 *func = new TF2("fitf",fitf,0.,6000.,0.0,0.2,npar);
  func->SetParameter(0,13.2);
  func->SetParameter(1,732.5);
  func->SetParameter(2,3.2e-07);
  func->SetParameter(3,-0.00442);
  func->SetParameter(4,13.2);
  h2->Fit(func,"");
  h2->Draw("LEGO2");
  for (i=0;i<npar;i++) {
    param[i] = func->GetParameter(i);
    printf (" parameter %d  %le  \n",i,param[i]);
  }
  for(i=0;i<n;i++) {
    ixy = h2->FindBin(x[i],y[i]);
    ezfit[i] =  h2->GetBinError(ixy);
    printf("i=%d  x=%f y=%f z=%f ezfit=%f  \n",i,x[i],y[i],z[i],ezfit[i]);
  }
  for(i=0;i<n;i++) {
    zzfit[i] = (param[0]*y[i]+param[1]*y[i]*y[i])*
     (TMath::Exp(param[2]*x[i]*x[i]+param[3]*x[i]+param[4]));
  }
// calcul des residus
  ressum = zero;
  for(i=0;i<n;i++) {
    aa     = (zzfit[i]-z[i])/ezfit[i];
    res    = aa*aa;
    printf ("x=%f y=%f z=%g zzfit=%g res=%g \n",x[i],y[i],z[i],zzfit[i],res);
    ressum += res;
  }
  chi2 = ressum/(n-npar);
  printf ("ressum= %g \n",ressum);
  printf ("chi2  = %g \n",chi2);
}