NovikovO. A., and RukasovV. I. “Approximation of Classes of Continuous Functions by Generalized De La Vallée-Poussin Sums”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 8, Aug. 1995, pp. 1069–1079, https://umj.imath.kiev.ua/index.php/umj/article/view/5503.