Abdullayev, F. G., and Y. I. Kharkevych. “Approximation of the Classes $C^{\psi}_{\beta}H^{\alpha}$ by Biharmonic Poisson Integrals”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 72, no. 1, Jan. 2020, pp. 20-35, https://umj.imath.kiev.ua/index.php/umj/article/view/1007.