“…Before 1970, Mullin and Rota in [26] introduced a simpler and more convenient operator method, umbral calculus, which is a linear-algebraic theory used to study certain types of polynomial functions that play an important role in applied mathematics [3,18]. Preservation of operators belong to theoretical basis of CAGD, involving preservation of smoothness, preservation of shape, one-side approximation, variation diminishing, the best constants in approximation of preservation and so on, on which there has been considerable research in [1,3,8,16,18,20,40]. Here we would also like to mention Refs.…”