@Article{ 
  ButSte3,
  author = { Buttazzo, Giuseppe and Stepanov, Eugene },
  title = { On regularity of transport density in the Monge-Kantorovich problem },
  journal = { SIAM J. Control Optim. },
  year = { 2003 },
  doi = { 10.1137/S036301290138568X },
  volume = { 42 },
  number = { 3 },
  pages = { 1044-1055 },
  URL = { http://cvgmt.sns.it/paper/3613/ },
  note = { cvgmt preprint}
}