@Misc{ 
  pra03,
  author = { Pratelli, Aldo },
  title = { Existence of optimal transport maps and regularity of the transport density in mass transportation problems },
  journal = {  },
  year = { 2003 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/758/ },
  note = { cvgmt preprint}
}