1. P. P. Korovkin, “Converging sequences of linear operators,”Uspekhi Mat. Nauk [Russian Math. Surveys],17, No. 4 (106), 147–152 (1962).
2. P. P. Korovkin, “An asymptotic property of the positive methods of summation of Fourier series and the best approximation of the class Z2 by linear positive polynomial operators,”Uspekhi Mat. Nauk [Russian Math. Surveys],13, No. 6(84), 99–103 (1958).
3. P. P. Korovkin, “On the order of approximation of functions by linear polynomial operators of classS m ,” in:Studies in the Contemporary Problems of Constructive Function Theory [in Russian], Izd. Akad. Nauk Azer. SSR, Baku (1965), pp. 163–166.
4. A. I. Kovalenko, “On certain methods of summation of Fourier series,”Mat. Sb. [Math. USSR-Sb.],71 (113), No. 4, 598–616 (1966).
5. J. Szabados, “On convolution operators with kernels of finite oscillation,”Acta Math. Acad. Sci. Hung.,27, No. 1–2, 179–192 (1976).