Romanyuk, A. S., and V. S. Romanyuk. “Trigonometric and Orthoprojection Widths of Classes of Periodic Functions of Many Variables”.
Ukrains’kyi Matematychnyi Zhurnal 61, no. 10 (October 25, 2009): 1348–1366. Accessed April 14, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/3106.