Vakarchuk, S. B. “On the Best Polynomial Approximations of $2π$-Periodic Functions and Exact Values of $n$-Widths of Functional Classes in the Space $L_2$”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 12 (December 25, 2002): 1603–1615. Accessed April 26, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4199.