Let $L_n^k(x)$ be the Laguerre polynomial of type $k$. I need a reference of this forumla
$\sum_{n=0}^{\infty} \frac{n !}{(n+a) \Gamma(n+k+1)} L_n^k(x) L_n^k(y)=\frac{1}{\Gamma(k+1)} \Phi(a, k+1 ; x) \Gamma(a) \Psi(a, k+1 ; y) \quad($ for $0<x \leq y)$
where $\Phi(a, c ; z) =_1 F_1(a ; c ; z)$ is the confluent hypergeometric function
and $\Psi(a, c ; z)=\frac{\Gamma(1-c)}{\Gamma(1+a-c)} \Phi(a, c ; z)+\frac{\Gamma(c-1)}{\Gamma(a)} z^{1-c} \Phi(1+a-c, 1-c ; z)$ is the tricomi-function.
Thank you very much for any help