On peculiar properties of generating functions of some orthogonal polynomials

We prove that for |x| ⩽ |t| < 1, −1 < q ⩽ 1 and n ⩾ 0: Σ≥(ti)/(q)ihn+i( x|q) =hn(x|t,q) Σi≥0(ti)/(q)ihi(x|q), where hn(x|q) and hn(x|t, q) are respectively the so-called q-Hermite and the big q-Hermite polynomials, and (q)n denotes the so-called q-Pochhammer symbol. We prove similar equalities involving big q-Hermite and Al-Salam–Chihara polynomials, and Al-Salam–Chihara and the so-called continuous dual q-Hahn polynomials. Moreover, we are able to relate in this way some other 'ordinary' orthogonal polynomials such as, e.g., Hermite, Chebyshev or Laguerre. These equalities give a new interpretation of the polynomials involved and moreover can give rise to a simple method of generating more and more general (i.e. involving more and more parameters) families of orthogonal polynomials. We pose some conjectures concerning Askey–Wilson polynomials and their possible generalizations. We prove that these conjectures are true for the cases q = 1 (classical case) and q = 0 (free case), thus paving the way to generalization of Askey–Wilson polynomials at least in these two cases. (paper)