Serdyuk, A. S. “Best Approximations and Widths of Classes of Convolutions of Periodic Functions of High Smoothness”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 7 (July 25, 2005): 946–971. Accessed May 5, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3655.