RukasovV. I., and ChaichenkoS. O. “Approximation by De La Vallée-Poussin Operators on the Classes of Functions Locally Summable on the Real Axis”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 7, July 2010, pp. 968–978, https://umj.imath.kiev.ua/index.php/umj/article/view/2929.