[[1] R. A. DeVore, G. G. Lorentz, Constructive Approximation, Springer, Berlin, 1993.10.1007/978-3-662-02888-9]Search in Google Scholar
[[2] Z. Ditzian, V. Totik, Moduli of Smoothness, Springer, New York, 1987.10.1007/978-1-4612-4778-4]Search in Google Scholar
[[3] J. P. King, Positive linear operators which preserve x2, Acta Math. Hungar., 99 (2003), 203–208.10.1023/A:1024571126455]Search in Google Scholar