1. S. M. Nikol'skii, ?The asymptotic behavior of the remainder in the approximation of functions satisfying a Lipschitz condition by Fejer sums,? Izv. Akad. Nauk SSSR, Ser. Matem.,4, 501?508 (1940).
2. P. L. Butzer and R. J. Nessel, Fourier Analysis and Approximation, Vol. 1, One-dimensional Theory, Academic Press (1971).
3. I. P. Natanson, ?The order of approximation of a continuous 2?-periodic function using its Poisson integral,? Dokl. Akad. Nauk SSSR,72, 11?14 (1950).
4. R. Taberski, ?On singular integrals,? Ann. Polon. Math.,4, 249?268 (1958).
5. R. G. Mamedov, ?The order and the asymptotic value of the approximation of functions by positive linear operators,? Izv. Akad. Nauk AzerbSSR, Ser. Fiz.-Matem. Tekhn., No. 2, 27?39 (1961).