@Article{ 
  ambservit05,
  author = { Ambrosio, Luigi and Serra Cassano, Francesco and Vittone, Davide },
  title = { Intrinsic regular hypersurfaces in Heisenberg groups },
  journal = { J. Geom. Anal. },
  year = { 2006 },
  volume = { 16 },
  number = { 2 },
  pages = { 187-232 },
  URL = { http://cvgmt.sns.it/paper/711/ },
  note = { cvgmt preprint}
}