A. S. Fedorenko, “Approximation of classes $C_\infty ^{\bar \psi }$ by zygmund sumsby zygmund sums”, Ukr. Mat. Zhurn., vol. 52, no. 6, pp. 856–860, Jun. 2000, Accessed: Apr. 23, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4483