KonsevichN. M. 2001. “Best Orthogonal Trigonometric Approximations of Classes of Functions of Many Variables $L^{ψ}_{β, p}$”. Ukrains’kyi Matematychnyi Zhurnal 53 (1), 23-29. https://umj.imath.kiev.ua/index.php/umj/article/view/4217.