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, Vol. 54, no. 12, Dec. 2002, pp. 1603-15, https://umj.imath.kiev.ua/index.php/umj/article/view/4199.