Romanyuk, A. S. “Trigonometric and Linear Widths for the Classes of Periodic multivariable functions”. Ukrains’kyi Matematychnyi Zhurnal 69, no. 5 (May 25, 2017): 670-681. Accessed April 19, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/1726.