@Article{ 
  ButRufVel12,
  author = { Buttazzo, Giuseppe and Ruffini, Berardo and Velichkov, Bozhidar },
  title = { Shape Optimization Problems for Metric Graphs },
  journal = { ESAIM Control Optim. Calc. Var. },
  year = { 2014 },
  volume = { 20 },
  number = { 01 },
  pages = { 1--22 },
  URL = { http://cvgmt.sns.it/paper/1923/ },
  note = { cvgmt preprint}
}