void genera()
{
  
  using namespace std;
  gROOT->Reset();


  TFile* f = (TFile*)gROOT->FindObject("samples.root"); if (f) f->Close();
  f = new TFile ("samples.root","RECREATE","");

  TTree * t = new TTree("t", "Nuovo Tree");
  int a = 16807;
  int m = 2147483647;
  int c=0;   //generatore di tipo moltiplicativo
  int Nmax=10000;
  int z, i;

  double PI = 3.141592653;
  double u, u0, y,r, rsq;
  double csi = pow(10, -4);
  double bm, bm2;
  double mean=15, sigma=2;
  	
    t->Branch("u", &u, "u/D");
	t->Branch("exp", &y, "y/D");
	t->Branch("BM", &bm, "bm/D");
	t->Branch("BM2", &bm2, "bm2/D");

	int z0 = 12; //seme

	for(i=0; i< Nmax; i++){
	z = (a*z0 +c) % m ;
	u = fabs((double)z/(double)m);  //distribuzione uniforme
	u0 = fabs((double)z0/(double)m); 
	y = -csi*log(u); // distribuzione esponenziale 
	bm = sqrt(-2*log(u))*sin(2*PI*u0);  //Box-Muller
	bm2 = mean + sigma*sqrt(-2*log(u))*sin(2*PI*u0);//Box-Muller mu=15, sigma=2
	t->Fill();
	z0 = z;
	}
	
  f->cd();
  t->Write();
  f->Close();
}