@Misc{ 
  fraserser08,
  author = { Franchi, Bruno and Serapioni, Raul and Serra Cassano, Francesco },
  title = { Differentiability of intrinsic Lipschitz functions within Heisenberg groups },
  journal = {  },
  year = { 2008 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/573/ },
  note = { cvgmt preprint}
}