Voitenko, S. P. “Best Orthogonal Trigonometric Approximations of the Classes $B^{Ω}_{p,θ}$ of Periodic Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 11, Nov. 2009, pp. 1473-84, https://umj.imath.kiev.ua/index.php/umj/article/view/3115.