pdtr

Poisson distribution.

SYNOPSIS:

int k;
double m, y, pdtr();
y = pdtr(k, m);

DESCRIPTION:

Returns the sum of the first k terms of the Poisson distribution:

k j
-- -m m
> e --
-- j!
j=0

The terms are not summed directly; instead the incomplete gamma integral is employed, according to the relation:

y = pdtr(k, m) = igamc(k+1, m).

The arguments must both be positive.

ACCURACY:
See igamc().