VAKARCHUK, S. B. Generalized characteristics of smoothness and some extremе problems of the approximation theory of functions in the space $L_2 (R)$. I. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 70, n. 9, p. 1166–1191, 2018. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/1627. Acesso em: 5 dec. 2025.