@Misc{ 
  ButVel17a,
  author = { Buttazzo, Giuseppe and Velichkov, Bozhidar },
  title = { Spectral optimization problems for Schrödinger operators },
  journal = {  },
  year = { 2017 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/3471/ },
  note = { cvgmt preprint}
}