Zhyhallo, T. V., and Y. I. Kharkevych. “Approximation of (ψ, β)-Differentiable Functions by Poisson Integrals in the Uniform Metric”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 11, Nov. 2009, pp. 1497-15, https://umj.imath.kiev.ua/index.php/umj/article/view/3117.