KonsevichN. M. “Best Orthogonal Trigonometric Approximations of Classes of Functions of Many Variables $L^{ψ}_{β, p}$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 1, Jan. 2001, pp. 23-29, https://umj.imath.kiev.ua/index.php/umj/article/view/4217.