TYRYGIN, I. Ya. A Kolmogorov type criterion for the best approximating operator. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 1, p. 114–119, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5789. Acesso em: 14 apr. 2025.