Vakarchuk, S. B., and V. I. Zabutnaya. 2012. “On the Best Polynomial Approximation in the Space L2 and Widths of Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal 64 (8), 1025-32. https://umj.imath.kiev.ua/index.php/umj/article/view/2637.