Zhyhallo, K. M., and Yu. I. Kharkevych. 2002. “Approximation of Differentiable Periodic Functions by Their Biharmonic Poisson Integrals”. Ukrains’kyi Matematychnyi Zhurnal 54 (9), 1213-19. https://umj.imath.kiev.ua/index.php/umj/article/view/4160.