Kovtonyuk, D. A., I. V. Petkov, and V. I. Ryazanov. “On the Dirichlet Problem for the Beltrami Equations in Finitely Connected Domains”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 7, July 2012, pp. 932-44, https://umj.imath.kiev.ua/index.php/umj/article/view/2629.