Rukasov, V. I., & Silin, E. S. (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