@Article{ 
  celfermar06,
  author = { Cellina, Arrigo and Ferriero, Alessandro and Marchini, Elsa },
  title = { On the existence of solutions to a minum time control problems and applications to Fermat's principle and the Brachistochrone },
  journal = { Systems Control Lett. },
  year = { 2006 },
  volume = { 55 },
  pages = { 119-123 },
  URL = { http://cvgmt.sns.it/paper/673/ },
  note = { cvgmt preprint}
}