Vakarchuk, S. B., and A. N. Shchitov. 2004. “Best Polynomial Approximations in $L_2$ and Widths of Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal 56 (11): 1458-66. https://umj.imath.kiev.ua/index.php/umj/article/view/3858.