Zhyhallo, K. M., & Kharkevych, Y. I. (2001). On the Approximation of Functions of the Hölder Class by Triharmonic Poisson Integrals. Ukrains’kyi Matematychnyi Zhurnal, 53(6), 855-859. https://umj.imath.kiev.ua/index.php/umj/article/view/4306