1. Astala, K., Iwaniec, T., Martin, G.J.: Deformations of annuli with smallest mean distortion. Arch. Ration. Mech. Anal. 195, 899–921 (2010)
2. Gilbarg, D., Trudinger, N.: Elliptic Partial Differential Equations of Second Order, 2nd edn. Springer, Berlin (1977, 1983)
3. Heinz, E.: On certain nonlinear elliptic differential equations and univalent mappings. J. d’Anal. Math. 5, 197–272 (1956/1957)
4. Hélein, F.: Regularity of weakly harmonic maps between a surface and a Riemannian manifold. C. R. Acad. Sci. Paris I I(312), 591–596 (1991). (French)
5. Hélein, F.: On weakly harmonic maps and Noether harmonic maps from a Riemann surface into a Riemannian manifold. Partial Differ. Equ. 27, 175–181 (1992)