V. I. Ryazanov, U. Srebro, and E. Yakubov, “On the theory of the Beltrami equation”, Ukr. Mat. Zhurn., vol. 58, no. 11, pp. 1571–1583, Nov. 2006, Accessed: Feb. 16, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3555