Rukasov, V. I., and O. I. Stepanets. “Approximation Properties of the De La Vallée-Poussin Method”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 8, Aug. 2002, pp. 1100-25, https://umj.imath.kiev.ua/index.php/umj/article/view/4150.