@Misc{ 
  dangar01a,
  author = { Danielli, Donatella and Garofalo, Nicola },
  title = { Green functions in nonlinear potential theory in Carnot groups and the geometry of their level sets },
  journal = {  },
  year = { 2001 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/729/ },
  note = { cvgmt preprint}
}