Towards a q-analogue of the Kibble–Slepian formula in 3 dimensions

We study a generalization of the Kibble–Slepian (KS) expansion formula in 3 dimensions. The generalization is obtained by replacing the Hermite polynomials by the q-Hermite ones. If such a replacement would lead to non-negativity for all allowed values of parameters and for all values of variables ranging over certain Cartesian product of compact intervals then we would deal with a generalization of the 3-dimensional Normal distribution. We show that this is not the case. Nevertheless we indicate other applications of so-generalized KS formula. Namely we use it to sum certain kernels built of the Al-Salam–Chihara polynomials for the cases that were not considered by other authors. One of such kernels sums up to the Askey–Wilson density disclosing its new, interesting properties. In particular we are able to obtain a generalization of the 2-dimensional Poisson–Mehler formula. We also pose several open questions.