RukasovV. I., and ChaichenkoS. O. 2002. “Approximation of Analytic Periodic Functions by De La Vallée-Poussin Sums”. Ukrains’kyi Matematychnyi Zhurnal 54 (12), 1653-69. https://umj.imath.kiev.ua/index.php/umj/article/view/4204.