@Misc{ 
  Bra21,
  author = { Brasco, Lorenzo },
  title = { Convex duality for principal frequencies },
  journal = { Math. Eng. },
  year = { 2021 },
  pages = { 25 },
  URL = { http://cvgmt.sns.it/paper/5133/ },
  note = { cvgmt preprint}
}