RomanyukA. S. “Best Trigonometric and Bilinear Approximations for the Besov Classes of Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 8, Aug. 1995, pp. 1097–1111, https://umj.imath.kiev.ua/index.php/umj/article/view/5506.