Doronin, V. G., and A. A. Ligun. 2009. “Exact Constants in Jackson-Type Inequalities for $L_2$-Approximation on an Axis”. Ukrains’kyi Matematychnyi Zhurnal 61 (1), 92-98. https://umj.imath.kiev.ua/index.php/umj/article/view/3004.