Hi, i am trying to answer the following question:

Consider the following joint distribution:

P(X=n, Y=m)= (m+n)(e^-2x) x^(m+n)/(n+m)!

n

where m and n takes values in 0,1,2.......

(the first bit should read m+n choose n)

show that X and Y are independent but have the same distribution as each other.

I know I should find the marginal distributions of X and Y, but Im not sure how to do so in this case, in my notes i only have examples where the joint distribution looks geometric. If i were to do it that way, to work out the marginal distribution of X, would I sum the joint distribution from m=0 to infinity and then work on simplifying it?

Thanks for your help