@Misc{ 
  Man21,
  author = { Mantegazza, Carlo },
  title = { On $C^2$ umbilical hypersurfaces },
  journal = {  },
  year = { 2021 },
  pages = {  },
  URL = { http://cvgmt.sns.it/paper/5104/ },
  note = { cvgmt preprint}
}