Zhyhallo, K. M., and Y. I. Kharkevych. “Approximation of Conjugate Differentiable Functions by Biharmonic Poisson Integrals”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 3, Mar. 2009, pp. 333-45, https://umj.imath.kiev.ua/index.php/umj/article/view/3023.