On the degree of approximation by the operators of de la Vallée Poussin

In this paper we study the degree of approximation of functionsf inC 2 andC 2 1 by the operatorsV n ofde la Vallée Poussin. The quality of approximation is measured in terms of the modulus of continuity off andf respectively. Forn so-called exact constants of approximation are determined. Furthermore, the asymptotic behaviour of these constants is investigated asn