N. V. Derev’yanko, “Trigonometric widths of classes of periodic functions of many variables”, Ukr. Mat. Zhurn., vol. 64, no. 8, pp. 1041–1052, Aug. 2012, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2639