Re: [Bug-gsl] cdf_poisson_P

[Rhys Ulerich]

Hi Georgios,

> Just to clarify: the function I am talking about is gsl_cdf_poisson_P, Not
> gsl_ran_poisson_pdf that I mistakenly mentioned below. The function
> gsl_ran_poisson_pdf (unsigned int k, double mu) is fine, and
> gsl_ran_poisson_pdf(-1,5) returns the expected zero.

According to
http://www.gnu.org/software/gsl/manual/html_node/The-Poisson-Distribution.html
gsl_cdf_poisson_P also takes an unsigned int as the first argument.
Passing negative values to the function is impossible because the
first argument is unsigned.

That "gsl_ran_poisson_pdf(-1,5) returns the expected zero" is mere
happenstance.  Somehow feeding the function "(unsigned) -1" ==
4294967295 (according to my current system) is producing the zero.

- Rhys