@Article{ 
  butpraste,
  author = { Buttazzo, Giuseppe and Pratelli, Aldo and Stepanov, Eugene },
  title = { Optimal pricing policies for public transportation networks },
  journal = { SIAM J. Opt },
  year = { 2006 },
  volume = { 16 },
  number = { 3 },
  pages = { 826-853 },
  URL = { http://cvgmt.sns.it/paper/247/ },
  note = { cvgmt preprint}
}