Vakarchuk, S. B., and A. N. Shchitov. “Best Polynomial Approximations in $L_2$ and Widths of Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal, vol. 56, no. 11, Nov. 2004, pp. 1458-66, https://umj.imath.kiev.ua/index.php/umj/article/view/3858.