1. Anastassiou G.A., Duman O., A Baskakov type generalization of statistical Korovkin theory, J. Math. Anal. Appl., 2008, 340, 476–486
2. Anastassiou G.A., Gal S.G., Approximation theory: Moduli of continuity and global smoothness preservation, Birkhäuser, Boston, 2000
3. Badea I., Modulus of continuity in Bögel sense and some applications for approximation by a Bernstein-type operator, Stud. Univ. Babeş-Bolyai Math., 1973, 18, 69–78 (in Romanian)
4. Badea C., Badea I., Cottin C., A Korovkin-type theorem for generalizations of Boolean sum operators and approximation by trigonometric pseudopolynomials, Anal. Numér. Théor. Approx., 1988, 17, 7–17
5. Badea C., Badea I., Gonska H.H., A test function and approximation by pseudopolynomials, Bull. Austral. Math. Soc., 1986, 34, 53–64