@Misc{ 
  big09,
  author = { Bigolin, Francesco },
  title = { Intrinsic regular hypersurfaces in Heisenberg groups and weak solutions of non linear first-order PDEs },
  journal = {  },
  year = { 2009 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/1268/ },
  note = { cvgmt preprint}
}