Stasyuk, S. A. “Best M-Term Orthogonal Trigonometric Approximations of the Classes B Ωp,θ of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal 60, no. 5 (May 25, 2008): 647 – 656. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/3183.