VakarchukS. B., and ShchitovA. N. “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, Vol. 58, no. 3, Mar. 2006, pp. 303-16, https://umj.imath.kiev.ua/index.php/umj/article/view/3455.