This is a curiosity I when looking at the binomial theorem. Say you have an ordinary generating function $\displaystyle\sum_{n,m\geq 0}\binom{n}{m}x^ny^m$. This looks kind of like $\displaystyle\sum_{i=0}^n\binom{n}{i}x^iy^{n-i}=(x+y)^n$.
Is there likewise a nice expression for the first generating function?
Of course, $$ \sum_{n,m \ge 0} \binom{n}{m} x^n y^m = \sum_{n=0}^\infty x^n \sum_{m=0}^n \binom{n}{m} y^m = \sum_{n=0}^\infty x^n (1+y)^n = \frac{1}{1-x(1+y)} $$