Babenko, V. F., V. G. Doronin, A. A. Ligun, and A. A. Shumeiko. “On Jackson-Type Inequalities for Functions Defined on a Sphere”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 3 (March 25, 2005): 291–304. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/3598.