Zhyhallo, K. M., and Yu. I. Kharkevych. 2000. “On the Approximation of Functions of the Hölder Class by Biharmonic Poisson Integrals”. Ukrains’kyi Matematychnyi Zhurnal 52 (7): 971-74. https://umj.imath.kiev.ua/index.php/umj/article/view/4496.