@Misc{ 
  ButvanVel14,
  author = { Buttazzo, Giuseppe and van den Berg, Michiel and Velichkov, Bozhidar },
  title = { Optimization problems involving the first Dirichlet eigenvalue and the torsional rigidity },
  journal = {  },
  year = { 2014 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/2378/ },
  note = { cvgmt preprint}
}