void poisson()
{
  const int NMAX = 20;

  TH1* h[4] = { 0 }; 
  double nu[4] = { 0.5, 2, 5, 10 };
  TF1* f = new TF1( "f", "gaus",0,20 );

  for( int i = 0; i < 4; ++i ) {
    h[i] = new TH1F( Form( "hpoisson%d", i ), 
		     Form( "Poisson Probability: #nu = %3.1f", nu[i] ), 
		     NMAX+1, -0.5, NMAX+0.5 );
    h[i]->SetXTitle( "n" );
    h[i]->SetYTitle( "P(n;#nu)" );
    h[i]->SetStats(0);
    for( int n = 0; n <= NMAX; ++n ) {
      h[i]->SetBinContent( n+1, TMath::PoissonI( n, nu[i] ) );
    }
    h[i]->Draw();

    f->SetParameters( TMath::PoissonI( nu[i],nu[i] ), nu[i], sqrt(nu[i]));
    f->Draw( "same" );

    gPad->SaveAs( Form( "poisson%d.pdf", i ) );
  }
}