@Article{ 
  vanButPra19,
  author = { Buttazzo, Giuseppe and Pratelli, Aldo and van den Berg, Michiel },
  title = { On the relations between principal eigenvalue and torsional rigidity },
  journal = { Comm. Contemp. Math. },
  year = { 2021 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/4514/ },
  note = { cvgmt preprint}
}