Rukasov, V. I., and E. S. Silin. “Approximation of Continuous Functions by De La Vallee-Poussin Operators”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 2 (February 25, 2005): 230–238. Accessed May 19, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3590.