Vakarchuk, S. B. (2002). 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(12), 1603-1615. https://umj.imath.kiev.ua/index.php/umj/article/view/4199