Vakarchuk, S. B., and V. I. Zabutnaya. “On the Best Polynomial Approximation in the Space L2 and Widths of Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 8, Aug. 2012, pp. 1025-32, https://umj.imath.kiev.ua/index.php/umj/article/view/2637.