1. A. I. Stepanets, Methods of Approximation Theory [in Russian], Vol. 2, Institute of Mathematics, Ukrainian National Academy of Sciences, Kiev (2002).
2. A. I. Stepanets, “Classes of functions defined on the real axis and their approximation by entire functions. I,” Ukr. Mat. Zh., 42, No. 1, 102–112 (1990).
3. A. I. Stepanets, “Classes of functions defined on the real axis and their approximation by entire functions. II,” Ukr. Mat. Zh., 42, No. 2, 210–222 (1990).
4. N. I. Akhiezer, Lectures on Approximation Theory [in Russian], Nauka, Moscow (1965).
5. P. P. Korovkin, “On the best approximation of functions of the class Z 2 by some linear operators,” Dokl. Akad. Nauk SSSR, 127, No. 3, 143–149 (1959).