Lambek, J., & Scott, P. J. (1981). Algebraic aspects of topos theory. Cahiers De Topologie Et Géométrie Différentielle Catégoriques, 22(2), 129–140. http://www.numdam.org/item/CTGDC_1981__22_2_129_0/ 
@article{CTGDC_1981__22_2_129_0,
  author = {Lambek, J. and Scott, P. J.},
  title = {Algebraic aspects of topos theory},
  journal = {Cahiers de Topologie et G\'eom\'etrie Diff\'erentielle Cat\'egoriques},
  pages = {129--140},
  publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS},
  volume = {22},
  number = {2},
  year = {1981},
  zbl = {0473.03064},
  mrnumber = {649793},
  language = {en},
  url = {http://www.numdam.org/item/CTGDC_1981__22_2_129_0/}
}