1. Asymptotic approximation of differentiable functions with linear positive operators;Mamedov;D.A.N.,1959
2. On some linear positive operators;Sikkema;Indag. Math.,1970
3. Voronovskaja, E. V., Determination of the asymptotic form of approximation of functions by the polynomials of S. N. Bernstein. Dokl. Akad. Nauk. SSSR, A, 79–85 (in Russian).