Double_t CauchyDens(Double_t *x, Double_t *par)
{
   Double_t pi   = TMath::Pi();
   Double_t mean = par[0];
   Double_t fwhm = par[1];

   Double_t arg = x[0]-mean;
   Double_t top = 0.5*fwhm;
   Double_t bot = pi*(arg*arg+top*top);

   Double_t func = top/bot;
   return func;
}

Double_t CauchyPeak(Double_t *x, Double_t *par)
{
   Double_t height = par[2];
   Double_t func = height*CauchyDens(x,par);
   return func;
}


void doit()
{
   TCanvas *c1 = new TCanvas("c1","the fit canvas",500,400);

// Creates a Root function based on function CauchDens above
   TF1 *func = new TF1("cauchy",CauchyDens,-10,10,2);

// Sets initial values and parameter names
   Double_t par[2];
   par[0] = 1.0; par[1] = 2.0;
   func->SetParameters(par);
   func->SetParNames("Mean","FWHM");
// Increase the number of points for calculating the integral of the density
   func->SetNpx(1000);

// Create a histogram filled between its boundaries with a
// Cauchy distribution
   TH1D *hCauchy = new TH1D("h","test",100,-5.,5.);
   hCauchy->Sumw2();
   Int_t nrPoints = 1000;
   hCauchy->FillRandom("cauchy",nrPoints);
   hCauchy->Draw();

// Extract the Cauchy density parameters from the fit
   TF1 *fitfunc = new TF1("fitfunc",CauchyPeak,-10,10,3);
   Double_t fitpar[3];
   fitpar[0] = par[0];
   fitpar[1] = par[1];
   fitpar[2] = Double_t(nrPoints);
   fitfunc->SetParameters(fitpar);
   hCauchy->Fit(fitfunc);
}