The point of the empty set
Cahiers de Topologie et Géométrie Différentielle Catégoriques, Volume 13 (1972) no. 4, p. 357-368
@article{CTGDC_1972__13_4_357_0,
     author = {Barr, Michael},
     title = {The point of the empty set},
     journal = {Cahiers de Topologie et G\'eom\'etrie Diff\'erentielle Cat\'egoriques},
     publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS},
     volume = {13},
     number = {4},
     year = {1972},
     pages = {357-368},
     zbl = {0258.18003},
     mrnumber = {323861},
     language = {en},
     url = {http://www.numdam.org/item/CTGDC_1972__13_4_357_0}
}
Barr, Michael. The point of the empty set. Cahiers de Topologie et Géométrie Différentielle Catégoriques, Volume 13 (1972) no. 4, pp. 357-368. http://www.numdam.org/item/CTGDC_1972__13_4_357_0/

M. Barr, (a) Coalgebras in a category of algebras, Lecture Notes in Math. 86 (1969), 1-12. | MR 244334 | Zbl 0211.03502

(b) Exact categories, Lecture Notes in Math. 236 (1971), 1-20. | Zbl 0221.18006

G. Grätzer, Universal Algebra, D. Van Nostrand & Co., 1968. | MR 248066 | Zbl 0182.34201

S. Mac Lane, Categories for the Working Mathematician, Springer-Verlag 1971. | MR 354798 | Zbl 0232.18001