Guitart, R. (1981). Carrés exacts et carrés déductifs. Diagrammes, 6. http://www.numdam.org/item/DIA_1981__6__A2_0/ 
@article{DIA_1981__6__A2_0,
  author = {Guitart, Ren\'e},
  title = {Carr\'es exacts et carr\'es d\'eductifs},
  journal = {Diagrammes},
  note = {talk:2},
  publisher = {Universit\'e Paris 7, Unit\'e d'enseignement et de recherche de math\'ematiques},
  volume = {6},
  year = {1981},
  zbl = {0499.18002},
  mrnumber = {684532},
  language = {fr},
  url = {http://www.numdam.org/item/DIA_1981__6__A2_0/}
}