@Article{ 
  pra06,
  author = { Pratelli, Aldo },
  title = { On the sufficiency of $c$-cyclical monotonicity for optimality of transport plans },
  journal = { Math. Z. },
  year = { 2007 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/147/ },
  note = { cvgmt preprint}
}