VakarchukS. B., and ZabutnayaV. I. “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.