BabenkoV. F., DoroninV. G., LigunA. A., and ShumeikoA. A. “On Jackson-Type Inequalities for Functions Defined on a Sphere”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 3, Mar. 2005, pp. 291–304, https://umj.imath.kiev.ua/index.php/umj/article/view/3598.