Serdyuk, A. S., and O. I. Stepanets. “Direct and Inverse Theorems in the Theory of Approximation of Functions in the Space $S^p$”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 54, no. 1, Jan. 2002, pp. 106-24,
https://umj.imath.kiev.ua/index.php/umj/article/view/4044.