Serdyuk, A. S., and O. I. Stepanets. 2002. “Direct and Inverse Theorems in the Theory of Approximation of Functions in the Space $S^p$”. Ukrains’kyi Matematychnyi Zhurnal 54 (1), 106-24. https://umj.imath.kiev.ua/index.php/umj/article/view/4044.