Michael Makkai, Robert Paré: Accessible categories: the foundations of categorical model theory. Contemporary Mathematics, 104. American Mathematical Society, Providence, RI, 1989. viii+176 pp.ISBN0-8218-5111-X, doi:10.1090/conm/104
M. Makkai: Duality and Definability in First Order Logic, Memoirs of the American Mathematical Society, 503, 1993, ISSN 0065-9266. doi:10.1090/memo/0503
↑M. Makkai, R. Paré: Accessible categories: the foundations of categorical model theory. Contemporary Mathematics, 104. American Mathematical Society, Providence, RI, 1989. viii+176 pp.ISBN0-8218-5111-X, doi:10.1090/conm/104
↑Paul Erdős, Michael Makkai: Some Remarks on Set Theory, X, Stud. Sci. Math. Hungar. 1, 157-159 (1966)