Hi,

I have the following problem.

Let X and Y be independent, with X~Poi and Y~Poi . Let Z = X + Y. Find the distribution X | Z = z.

Now I know that the sum of two independent random variables which both have a Poisson distribution, also has a Poisson distribution.

so the pmf of Z is

and

but Im having trouble calculating . This is where Im stuck.

I know that the conditional probability will return a binomial distribution. Can anyone please help me understand how to calculate and consequently obtain a binomial distribution?