the function Double_t TMath::Poisson(Double_t x, Double_t par) { // compute the Poisson distribution function for (x,par) if (x<0) return 0; if (x==0) return TMath::Exp(-par); return TMath::Power(par,x)/TMath::Gamma(x+1)/TMath::Exp(par); } dosn't appear to work for x=0.... cannot imagine what the problem is! root [28] TMath::Poisson(0.,2.) (Double_t)0.00000000000000000e+00 root [29] TMath::Poisson(1.,2.) (Double_t)2.70670566473225516e-01 root [30] TMath::Poisson(2.,2.) (Double_t)2.70670566473225516e-01 root [31] root [31] root [31] TMath::Power(2,0)/TMath::Gamma(0+1)/TMath::Exp(2) (double)1.35335283236612702e-01 root [32] TMath::Power(2,1)/TMath::Gamma(1+1)/TMath::Exp(2) (double)2.70670566473225516e-01 root [33] TMath::Power(2,2)/TMath::Gamma(2+1)/TMath::Exp(2) (double)2.70670566473225516e-01 -- Michael Gold Department of Physics and Astronomy University of New Mexico Albuquerque, NM 87131 cell : 505-238-6155 phone: 505-277-2086, 505-277-3604 fax: 505-277-1520 email: gold@phys.unm.edu
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