Zhyhallo, K. M., and Yu. I. Kharkevych. “On the Approximation of Functions of the Hölder Class by Triharmonic Poisson Integrals”. Ukrains’kyi Matematychnyi Zhurnal 53, no. 6 (June 25, 2001): 855-859. Accessed May 11, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4306.