[Bug-gsl] gsl_ran_poisson_pdf with mu=0

[Peter Johansson]

Hi,

I noticed that gsl_ran_poisson_pdf(k, mu) error out if mu=0.0. IMHO a 
Poisson distribution with zero mean is a perfectly valid distribition 
(although it is not very random), so I find it unexpected that the 
function doesn't support it. I suggest to add some code taking care of 
the special case. Something like:

if (!mu) {
  if (k)
    return 0.0
  return 1.0
}


Cheers,
Peter
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