“…An interesting property of these polynomials is that they do not form an orthogonal set with respect to the standard inner-products, but they are orthogonal with respect to some new inner-products involving differential or difference operators. These classes of polynomials (called Sobolev orthogonal polynomials) have been the object of much attention in the last years (see [4], [11], [12], [14], [18], [25], [27], [28], [29], [30], [33], [37], [39], [41], [44], [45], [46], [47], [49], [52]).…”