HrabovaU. Z., and SerdyukA. S. 2013. “Order Estimates for the Best Approximations and Approximations by Fourier Sums of the Classes of (ψ, β)-Differential Functions”. Ukrains’kyi Matematychnyi Zhurnal 65 (9), 1186–1197. https://umj.imath.kiev.ua/index.php/umj/article/view/2500.