@Misc{ 
  BelLeD12,
  author = { Bellettini, Costante and Le Donne, Enrico },
  title = { Regularity of sets with constant horizontal normal in the Engel group },
  journal = {  },
  year = { 2012 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/4166/ },
  note = { cvgmt preprint}
}