Tyrygin, I. Ya. “A Kolmogorov Type Criterion for the Best Approximating Operator”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 1 (January 25, 1993): 114–119. Accessed April 14, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5789.