@Misc{ 
  FerSil21,
  author = { Ferrari, Matteo and Sillari, Lorenzo },
  title = { On the minimal number of solutions of the equation $ φ(n+k)= M \,
  φ(n) $, $ M=1$, $2$ },
  journal = {  },
  year = { 2021 },
  pages = {  },
  URL = { https://gecogedi.dimai.unifi.it/paper/553/ },
  note = { cvgmt preprint}
}