ShvaiK. V. “Estimates of the Best Bilinear Approximations for the Classes of $(ψ,β)$-Differentiable Periodic Multivariate Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 4, Apr. 2018, pp. 564-73, http://umj.imath.kiev.ua/index.php/umj/article/view/1576.