@Misc{ 
  fraserser04,
  author = { Franchi, Bruno and Serapioni, Raul and Serra Cassano, Francesco },
  title = { Regular submanifolds, graphs and area formula in Heisenberg groups },
  journal = { Advances in Mathematics },
  year = { 2004 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/967/ },
  note = { cvgmt preprint}
}