The nonlinear membrane energy: variational derivation under the   constraint "det\nabla u>0"
  In [4] we gave a variational definition of the nonlinear membrane energy under the constraint "det\nabla u\not=0". In this paper we obtain the nonlinear membrane energy under the more realistic constraint "det\nabla u>0". 
Omar Anza Hafsa, Jean-Philippe Mandallena