Doronin, V. G., & Ligun, A. A. (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