Vakarchuk, S.B. and Zabutnaya, V.I. (2012) “On the best polynomial approximation in the space L2 and widths of some classes of functions”, Ukrains’kyi Matematychnyi Zhurnal, 64(8), pp. 1025–1032. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/2637 (Accessed: 5 December 2025).