1. [1] Korovkin B., On convergence of linear positive operators in the space of continuous functions, Doklady Akad. Nauk SSSR (N.S.), 1953, 90, 961–964
2. [2] Wolff M., On the universal Korovkin closure of subsets in vector lattices, J. Approx. Theory, 1978, 22, 243–253
3. [3] Altomare F., Korovkin-type and approximation by positive linear operators, Surv. Approx. Theory, 2010, 5, 92–164
4. [4] Duman O., Orhan C., An abstract version of the Korovkin approximation theorem, Publ. Math. Debrecen, 2006, 69, 33–46
5. [5] Guessab A., Schmeisser G., Two Korovkin-type theorems in multivariate approximation, Banach J. Math. Anal., 2008, 2, 221–128