D. M. Bushev and I. R. Koval'chuk, “On approximation of convolution classes”, Ukr. Mat. Zhurn., vol. 45, no. 1, pp. 26–31, Jan. 1993, Accessed: Apr. 13, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5778