Serdyuk, A. S. “Approximation of Poisson Integrals by De La Valleé-Poussin Sums in Uniform and Integral Metrics”. Ukrains’kyi Matematychnyi Zhurnal, vol. 62, no. 12, Dec. 2010, pp. 1672–1686, https://umj.imath.kiev.ua/index.php/umj/article/view/2991.