1. Ditzian, Z. and Totik, V., Moduli of Smoothness, Springer-Verlag, New York Berlin Heidelberg 1987.
2. Felten, M., Local and Global Approximation Theorems for Positive Linear Operators, J. Approx. Theory, 94(1988), 396–419.
3. Guo, S. S., Li, C. X. and Zhang, G. S., Pointwise Estimate for Baskakov Operators, Northeast. Math. J., 17:2(2001), 134–137.
4. Guo, S. S., Li, C. X., Sun, Y. G., Yang, G. and Yue, S. J., Pointwise Estimate for Szász-type Operators, J. Approx. Theory, 94(1998), 160–171.
5. Heilmann, M., Direct and Converse Results for Operators of Baskakov-Durrmeyer type, Approx. Theory & its Appl., 5:1(1989), 105–127.