RomanyukA. S., and RomanyukV. S. “Estimates for the Best Bilinear Approximations of the Classes $B^r_{p,\theta}$ and Singular Numbers of Integral Operators”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 9, Sept. 2016, pp. 1240-5, https://umj.imath.kiev.ua/index.php/umj/article/view/1917.