@Misc{ 
  bergazmit05,
  author = { Berchio, Elvise and Gazzola, Filippo and Mitidieri, Enzo },
  title = { Positivity preserving property for a class of biharmonic elliptic problems },
  journal = {  },
  year = { 2005 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/1285/ },
  note = { cvgmt preprint}
}