VakarchukB. S. (2016). Jackson-type inequalities with generalized modulus of continuity and exact values of the $n$-widths for the classes of $(ψ,β)$-differentiable functions in $L_2$. III. Ukrains’kyi Matematychnyi Zhurnal, 68(10), 1299-1319. Retrieved from http://umj.imath.kiev.ua/index.php/umj/article/view/1922