RukasovV. I., & ChaichenkoS. O. (2001). Approximation of $\overline \psi$-Integrals of Periodic Functions by de la Vallée-Poussin Sums (Low Smoothness). Ukrains’kyi Matematychnyi Zhurnal, 53(12), 1641-1653. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/4384