Rukasov, V. I., and E. S. Silin. 2005. “Approximation of Continuous Functions of Low Smoothness by De La Vallee-Poussin Operators”. Ukrains’kyi Matematychnyi Zhurnal 57 (3), 394–399. https://umj.imath.kiev.ua/index.php/umj/article/view/3607.