K. M. Zhyhallo and Y. I. Kharkevych, “Approximation of (ψ, β)-differentiable functions of low smoothness by biharmonic Poisson integrals”, Ukr. Mat. Zhurn., vol. 63, no. 12, pp. 1602–1622, Dec. 2011, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2829