DORONIN, V. G.; LIGUN, A. A.; SERDYUK, A. S.; SHYDLICH, A. L. Exact constants in Jackson-type inequalities for $L_2$-approximation on an axis. Ukrains’kyi Matematychnyi Zhurnal, v. 61, n. 1, p. 92-98, 25 Jan. 2009.