Hembars'ka, S. B., and K. M. Zhyhallo. 2017. “Approximative Properties of Biharmonic Poisson Integrals on Hölder Classes”. Ukrains’kyi Matematychnyi Zhurnal 69 (7): 925-32. https://umj.imath.kiev.ua/index.php/umj/article/view/1746.