“…The orthogonality of some classical systems of polynomials, in the outstanding situation in which the three term recurrence relation breaks down (so the hypothesis of Favard's theorem does not hold), has been successfully developed in the last decade: see [21,24] for the Laguerre case, [22,4,2,1] for the Jacobi case, [3,12] for the Meixner case, [5,6] for the case of symmetric Meixner-Pollaczek polynomials, [13] for (not necessarily symmetric) Meixner-Pollaczek polynomials with null parameter and, finally, [12] for the classical families of polynomials which satisfy a discrete orthogonality with a finite number of masses (i.e., the Hahn, Racah, dual Hahn and Krawtchouk polynomials).…”