Shvai, K. V. “Estimates of the Best Bilinear Approximations for the Classes of $(ψ,β)$-Differentiable Periodic Multivariate Functions”. Ukrains’kyi Matematychnyi Zhurnal 70, no. 4 (April 25, 2018): 564–573. Accessed April 14, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/1576.