Derev’yanko, N.V. (2012) “Trigonometric widths of classes of periodic functions of many variables”, Ukrains’kyi Matematychnyi Zhurnal, 64(8), pp. 1041–1052. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/2639 (Accessed: 6 December 2025).