Voitenko, S. P. “Best Orthogonal Trigonometric Approximations of the Classes $B^{Ω}_{p,θ}$ of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 11 (November 25, 2009): 1473-1484. Accessed May 2, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3115.