VakarchukS. B., and ShchitovA. N. “Best Polynomial Approximations in $L_2$ and Widths of Some Classes of Functions”. Ukrains’kyi Matematychnyi Zhurnal 56, no. 11 (November 25, 2004): 1458-1466. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3858.