1. A. I. Stepanets, Methods of Approximation Theory. I [in Russian], Inst. Math. of the NASU, Kiev (2002).
2. A. I. Stepanets and E. I. Zhukina, “Inverse theorems of approximation of (ψ, β)-differentiable functions,” Ukr. Mat. Zh., 41, No. 86, 1106–1112 (1989).
3. A. F. Timan, Theory of Approximation of Functions of a Real Variable, Dover, New York (1964).
4. A. A. Konyushkov, “The best approximations by trigonometric polynomials and Fourier coefficients,” Mat. Sborn., 44, No. 86, 53–84 (1958).
5. K. Yano, “On Hardy and Littlewood’s theorem,” Proc. Japan Acad., 33, No. 2, 73–74 (1957).