SerdyukA. S. “Widths and Best Approximations for Classes of Convolutions of Periodic Functions”. Ukrains’kyi Matematychnyi Zhurnal 51, no. 5 (May 25, 1999): 674–687. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4653.