An extension of Reshetnyak's theorem Juan ManfrediEnrique Villamor Quasiregular mappingsdegenerate elliptic equationsnonlinear elasticity Indiana University Mathematics Journal 1998 text pdf 10.1512/iumj.1998.47.1323 10.1512/iumj.1998.47.1323 en Indiana Univ. Math. J. 47 (1998) 1131 - 1146 state-of-the-art mathematics http://iumj.org/access/