NOVIKOV, O. A.; RUKASOV, V. I. Approximation of the classes $C_{β}^{ψ} H_{ω}$ by generalized de la Valiée-Poussin sums. Ukrains’kyi Matematychnyi Zhurnal, v. 49, n. 4, p. 606-610, 25 Apr. 1997.