Myronyuk, V. V. 2014. “Trigonometric Approximations and Kolmogorov Widths of Anisotropic Besov Classes of Periodic Functions of Several Variables”.
Ukrains’kyi Matematychnyi Zhurnal 66 (8): 1117–1132.
https://umj.imath.kiev.ua/index.php/umj/article/view/2203.