@Article{ 
  gbuest03,
  author = { Buttazzo, Giuseppe and Stepanov, Eugene },
  title = { Optimal transportation networks as free Dirichlet regions for the Monge-Kantorovich problem },
  journal = { Ann. Sc. Norm. Sup. Pisa Cl. Sci. },
  year = { 2003 },
  volume = { II },
  number = { 4 },
  pages = { 631-678 },
  URL = { http://cvgmt.sns.it/paper/661/ },
  note = { cvgmt preprint}
}