Hi,

I see what you are saying I think. I should have said the distribution of the sum of two independent random variables. But to find the distribution for the sum X+Y, i.e. the joint, one has to take the sum over the product of the two distributions (that's where independence comes in), right ? i.e. for Z = X +Y canonically

so does this translate for our example as

?

But I cant massage this form into the desired form. Am I setting things up incorrectly here?

And here the notation implies that k is the same for both initial distributions. Should the notation not more correctly be

and

So that

Or am I missing something (as I guess is much more likely) ?

Thanks again.