@article{Vai:62a,
  journal = {C. R. Acad. Bulgare Sci.},
  volume = {15},
  title = {Un probl\`eme de {G}. {B}irkhoff},
  author = {Vaida, D.},
  year = {1962},
  fjournal = {Doklady Bolgarskoi Akademii Nauk. Comptes Rendus de l'Acad\'emie
              Bulgare des Sciences},
  pages = {801--803},
}