RomanyukA. S. “Best Approximations of the Classes $B_{p,\,\theta}^{r}$ of Periodic Functions of Many Variables in Uniform Metric”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 10, Oct. 2006, pp. 1395–1406, https://umj.imath.kiev.ua/index.php/umj/article/view/3541.