1. D. Jackson, Über die Genauigkeit der Annäherung stetiger Funktionen durch ganze rationale Funktionen gegebenen Grades und trigonometrische Summer gegebener Ordnung: Inauguraldisertation und Preischrift, University of Göttingen (1911).
2. S. B. Stechkin, “On the order of the best approximations of continuous functions,” Izv. Akad. Nauk SSSR, Ser. Mat., 15, No. 3, 219–242 (1951).
3. N. I. Akhiezer, Lectures on the Theory of Approximations [in Russian], Gostekhteorizdat, Moscow-Leningrad (1947).
4. G. V. Radzievskii, “The rate of convergence of decompositions of ordinary functional-differential operators by eigenfunctions,” in: Some Problems of the modern Theory of Differential equations, Preprint No. 94.29, Institute of Mathematics, Ukrainian Academy of Sciences, Kiev (1994), pp. 14–27.
5. G. V. Radzievskii, “Boundary-value problems and related moduli of continuity,” Funkts. Anal. Prilozh., 29, No. 3, 87–90 (1995).