void pdf_cdf() 
{
  TH1* hpp = new TH1F( "hpoissonPDF", "Poisson PDF", 110, -1, 10 );
  hpp->SetStats( 0 );
  TH1* hpc = new TH1F( "hpoissonCDF", "Poisson CDF", 11, -1, 10 );
  hpc->SetStats( 0 );

  for( int i = 0; i < 10; ++i ) {
    hpp->Fill( i, TMath::PoissonI( i, 3 ) );
    hpc->SetBinContent( i+2, hpc->GetBinContent( i+1 ) + TMath::PoissonI( i, 3 ) );
  }
  hpp->Draw(); 
  gPad->SaveAs( "poisson_pdf.pdf" );

  hpc->Draw(); 
  gPad->SaveAs( "poisson_cdf.pdf" );

  TF1* fgp = new TF1( "fgp", "TMath::Gaus(x,0,1,1)", -5, 5 );
  fgp->Draw();
  fgp->SetTitle( "Gaussian PDF" );
  gPad->SaveAs( "gaussian_pdf.pdf" );

  TF1* fgc = new TF1( "fgc", "TMath::Freq(x)", -5, 5 );
  fgc->Draw();
  fgc->SetTitle( "Gaussian CDF" );
  gPad->SaveAs( "gaussian_cdf.pdf" );

}