Doronin , V. G., and A.A. Ligun. “Problem of Amplification of the Jackson-Type Inequalities ”. Ukrains’kyi Matematychnyi Zhurnal 44, no. 4 (April 17, 1992): 475–480. Accessed April 11, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/7888.