@Misc{ 
  beopao07,
  author = { Bellettini, Giovanni and Beorchia, Valentina and Paolini, Maurizio },
  title = { An explicit formula for a Bennequin-type invariant for apparent contours },
  journal = {  },
  year = { 2007 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/35/ },
  note = { cvgmt preprint}
}