1. R. A. DeVore,The approximation of continuous functions by positive linear operators, Springer (Berlin-Heidelberg-New York, 1972).
2. L.Fejér, Über Interpolation,Göttinger Nachrichten (1916), 66–91.
3. P. P. Korovkin,Linear operators and approximation theory, Hindustan Pub. Corp. (Delhi, 1960).
4. A. Ralston,A first course in numerical analysis, McGraw-Hill (New York-Toronto-London, 1965).
5. M. A. Snyder,Chebyshev methods in numerical approximation, Prentice-Hall (Englewood Cliffs, N. J., 1966).