@Article{ 
  barman09,
  author = { Bardi, Martino and Mannucci, Paola },
  title = { Comparison principles for equations of Monge-Ampère type in Carnot groups: a direct proof  },
  journal = { Lect. Notes Semin. Interdiscip. Mat. },
  year = { 2008 },
  volume = { 7 },
  pages = { 41-51 },
  URL = { http://cvgmt.sns.it/paper/421/ },
  note = { cvgmt preprint}
}