Double_t ud_pois_p6(Double_t*x,Double_t*par)                                         
{                                                                              

  Int_t n = 0;
  Double_t arg = 0;
  if (x==0) return TMath::Exp(-par[1]);   

  for(n = 0; n <100; n++)
    {
  
      arg += par[0]
           * ((TMath::Power(par[1],n)*TMath::Exp(par[1]))/TMath::Gamma(n+1)) 
           * (1.0/(sqrt(2.0*TMath::Pi())* (1+par[2]))) 
           * (TMath::Exp(-(x[0]-par[3]*n)*(x[0]-par[3]*n))/(2.0*(1+par[2])*(1+par[2])))
           + (1.0/(2.0 * TMath::Pi()* (1+par[4])))*(TMath::Exp(-(x[0]-par[5]*n)*(x[0]-par[5]*n))/(2.0*(1+par[4])*(1+par[4])))
           ;
    }
  return arg;

}