ZhyhalloT. V., and KharkevychYu. I. “Approximation of Functions from the Class $\hat{C}^{\psi}_{\beta, \infty}$ by Poisson Biharmonic Operators in the Uniform Metric”. Ukrains’kyi Matematychnyi Zhurnal 60, no. 5 (May 25, 2008): 669 -. Accessed November 23, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3185.