[1]
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.