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