Derev’yanko, N. V. “Trigonometric Widths of Classes of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal 64, no. 8 (August 25, 2012): 1041-1052. Accessed April 25, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2639.