T. V. Zhyhallo and Y. I. Kharkevych, “Approximating properties of biharmonic Poisson operators in the classes $\hat{L}^{\psi}_{\beta, 1}$ ”, Ukr. Mat. Zhurn., vol. 69, no. 5, pp. 650–656, May 2017, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/1724