@Article{ 
  Pra15a,
  author = { Pratelli, Aldo },
  title = { On the bi-Sobolev planar homeomorphisms and their approximation },
  journal = { Nonlinear Anal. },
  year = { 2017 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/2782/ },
  note = { cvgmt preprint}
}