1. B. E. Klots, “Approximation of differentiable functions by functions of higher smoothness,” Mat. Zametki, 21, No. 1, 21–32 (1977).
2. N. P. Korneichuk, V. F. Babenko, V. A. Kofanov, and S. A. Pichugov, Inequalities for Derivatives and Their Applications [in Russian], Naukova Dumka, Kiev (2003).
3. V. F. Babenko, V. A. Kofanov, and S. A. Pichugov, "Exact Kolmogorov-type inequalities with bounded leading derivative in the case of low smoothness," Ukr. Mat. Zh., 53, No. 10, 1298-1308 (2001)
4. English translation: Ukr. Math. J., 53, No. 10, 1569-1582 (2001).
5. V. F. Babenko, V. A. Kofanov, and S. A. Pichugov, "Comparison of exact constants in inequalities for derivatives of functions defined on the real axis and a circle," Ukr. Mat. Zh., 55, No. 5, 579-589 (2003)