Lomako, T. “Theorem on Closure and the Criterion of Compactness for the Classes of Solutions of the Beltrami Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 65, no. 12, Dec. 2013, pp. 1657–1666, https://umj.imath.kiev.ua/index.php/umj/article/view/2544.