Rukasov, V. I. “Approximation of Classes of Analytic Functions by De La Vallée-Poussin Sums”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 55, no. 6, June 2003, pp. 806-1, https://umj.imath.kiev.ua/index.php/umj/article/view/3955.