KovtonyukD. A., PetkovI. V., and RyazanovV. I. 2011. “On the Boundary Behavior of Solutions of the Beltrami Equations”. Ukrains’kyi Matematychnyi Zhurnal 63 (8), 1078-91. https://umj.imath.kiev.ua/index.php/umj/article/view/2786.