# Two-variable generating functions for Laguerre polynomials

Where can I find generating functions for orthogonal polynomials in two variables? Lebedev's book (Special Functions and their Applications, Dover, 1972) gives a closed form for $$\sum_{n=0}^\infty \frac{n!}{\Gamma(n+\alpha+1)} L_n^\alpha(x) L_n^\alpha(y) t^n$$ as an explicit function of $x,y,t$ (it involves a Bessel function). However, I need to sum over the upper index (when it is an integer), to find $$\sum_{k=0}^\infty \frac{n!}{(n+k)!} L_n^k(x) L_n^k(y) t^k = {??}$$ in a closed form $F(x,y,t)$, for a fixed positive integer $n$. Are generating functions of this kind to be found in the literature? (Not on DLMF, as far as I can see.) A closed form for the previous sum would be great, a pointer to a suitable article would be better.

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