@Misc{ 
  belpao,
  author = { Bellettini, Giovanni and Paolini, Maurizio },
  title = { On the area of the graph of a singular map from the plane to the plane taking three values },
  journal = {  },
  year = { 2009 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/563/ },
  note = { cvgmt preprint}
}