K. M. Zhyhallo and Y. I. Kharkevych, “On the Approximation of Functions of the Hölder Class by Triharmonic Poisson Integrals”, Ukr. Mat. Zhurn., vol. 53, no. 6, pp. 855–859, Jun. 2001, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4306