ZHYHALLO, T. V.; KHARKEVYCH, Y. I. Approximation of functions from the class $\hat{C}^{\psi}_{\beta, \infty}$ by Poisson biharmonic operators in the uniform metric. Ukrains’kyi Matematychnyi Zhurnal, v. 60, n. 5, p. 669 -, 25 May 2008.