Vakarchuk, S. B. (2018). Generalized characteristics of smoothness and some extreme problems of the approximation theory of functions in the space $L_2 (R)$. II. Ukrains’kyi Matematychnyi Zhurnal, 70(10), 1345-1373. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/1640