Hi,
Just in case anyone (else) encounters this, I thought I would post it to this list.
In Root 5.26.00/b,
root [9] ROOT::Math::poisson_pdf(0,0)
(double)nan
root [11] TMath::PoissonI(0,0)
(Double_t)1.00000000000000000e+00
I personally would have expected the poisson_pdf to give 1 (the chance to get 0 events in the limit that the mean goes to 0).
I don't know if this behavior is intended or not.
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