Radzievskaya, E. I., and G. V. Radzievskii. “Estimation of a K-Functional of Higher Order in Terms of a K-Functional of Lower Order”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 11 (November 25, 2003): 1530–1540. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4022.