Vakarchuk, S. B., and A. N. Shchitov. 2006. “On Some Extremal Problems in the Theory of Approximation of Functions in the Spaces $S^p,\quad 1 \leq P < \infty$”. Ukrains’kyi Matematychnyi Zhurnal 58 (3), 303-16. https://umj.imath.kiev.ua/index.php/umj/article/view/3455.