@Misc{ 
  LeD14,
  author = { Le Donne, Enrico },
  title = { A metric characterization of Carnot groups },
  journal = { Proceedings of the American Mathematical Society },
  year = { 2014 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/2505/ },
  note = { cvgmt preprint}
}