V. I. Rukasov and S. O. Chaichenko, “Approximation of $\overline \psi$-Integrals of Periodic Functions by de la Vallée-Poussin Sums (Low Smoothness)”, Ukr. Mat. Zhurn., vol. 53, no. 12, pp. 1641–1653, Dec. 2001, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4384