@Article{ 
  giatre05,
  author = { Giacomini, Alessandro and Trebeschi, Paola },
  title = { A density result for Sobolev spaces in dimension two, and applications to stability of nonlinear Neumann problems },
  journal = { J. Differential Equations },
  year = { 2007 },
  volume = { 237 },
  pages = { 27-60 },
  URL = { http://cvgmt.sns.it/paper/880/ },
  note = { cvgmt preprint}
}