V. I. Rukasov and S. O. Chaichenko, “Approximation of the Classes $C^{{\bar \psi }} H_{\omega }$ by de la Vallée-Poussin Sums”, Ukr. Mat. Zhurn., vol. 54, no. 5, pp. 681–691, May 2002, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4107