K. M. Zhyhallo and Y. I. Kharkevych, “Approximation of functions from the classes $C^{\psi}_{\beta, \infty}$ by biharmonic Poisson integrals”, Ukr. Mat. Zhurn., vol. 63, no. 7, pp. 939–959, Jul. 2011, Accessed: Apr. 09, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2776