On Askey-scheme and dd-orthogonality, I: A characterization theorem

This is the first in a series of papers dealing with generalized hypergeometric dd-orthogonal polynomials extending the polynomial families in the Askey-scheme. In this paper, we give a characterization theorem to introduce new examples of generalized hypergeometricdd-orthogonal polynomials to be studied in the forthcoming works. For d=1d=1, we obtain an unification of some known characterization theorems in the orthogonal polynomials theory.