K. V. Shvai, “Estimates of the best bilinear approximations for the classes of $(ψ,β)$-differentiable periodic multivariate functions”, Ukr. Mat. Zhurn., vol. 70, no. 4, pp. 564–573, Apr. 2018, Accessed: Apr. 13, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/1576