1. Ahlfors, L.V.: Lectures on Quasiconformal Mappings, 2nd edn. With supplemental chapters by C. J. Earle, I. Kra, M. Shishikura and J. H. Hubbard. University Lecture Series, 38. American Mathematical Society, Providence (2006)
2. Astala, K., Iwaniec, T., Martin, G.: Elliptic partial differential equations and quasiconformal mappings in the plane. Princeton University Press, NJ (2009)
3. Evans, L.C.: A new proof of local $$C^{1,\alpha }$$ regularity for solutions of certain degenerate elliptic P.D.E. J. Differ. Equ. 45, 356–373 (1982)
4. Gilbarg, D., Trudinger, N.: Elliptic Partial Differential Equations of Second Order, 2 Edn. Springer 1977 (1983)
5. Heinz, E.: On certain nonlinear elliptic differential equations and univalent mappings. J. Anal. Math. 5, 197–272 (1957)