Babenko, V. F., Doronin, V. G., Ligun, A. A., & Shumeiko, A. A. (2005). On Jackson-Type Inequalities for Functions Defined on a Sphere. Ukrains’kyi Matematychnyi Zhurnal, 57(3), 291–304. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3598