@Article{ 
  mag09,
  author = { Magnani, Valentino },
  title = { Nonexistence of horizontal Sobolev surfaces in the Heisenberg group },
  journal = { Proc. Amer. Math. Soc.  },
  year = { 2010 },
  volume = { 138 },
  number = { 5 },
  pages = { 1785–1791 },
  URL = { http://cvgmt.sns.it/paper/575/ },
  note = { cvgmt preprint}
}