@Misc{ 
  BalFra13,
  author = { Baldi, Annalisa and Franchi, Bruno },
  title = { Sharp a priori estimates for div-curl systems in Heisenberg groups },
  journal = {  },
  year = { 2013 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/2126/ },
  note = { cvgmt preprint}
}