Romanyuk, A. S., and V. S. Romanyuk. “Trigonometric and Orthoprojection Widths of Classes of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 10, Oct. 2009, pp. 1348-66, https://umj.imath.kiev.ua/index.php/umj/article/view/3106.