KovtonyukD. A., PetkovI. V., and RyazanovV. I. “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.