Let X1,...,Xn be a random sample from a Poisson distribution with mean theta. Find
I know that
I need to find
How would I find the joint PMF of Y1 and Y2?
I think the formula you use is only available with probability distributions with densities.
Anyway, look here : http://www.mathhelpforum.com/math-he...757-post2.html
which proves that
And then just write (linearity of the expectation)
And you're done.