@Misc{ 
  Rio18a,
  author = { Rios-Zertuche, Rodolfo },
  title = { Existence of differentiable curves in convex sets and the concept of
  direction of the flow in mass transportation },
  journal = {  },
  year = { 2018 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/4215/ },
  note = { cvgmt preprint}
}