1. E. F. Codd, “A relational model of data for large shared data banks”, Comm. ACM,13, 377–387 (1970).
2. W. Armstrong, “Dependency structures of data base relationships”, in: Information Processing '74, North-Holland, Amsterdam (1974), pp. 580–583.
3. J. Demetrovics and G. O. H. Katona, “Extremal combinatorial problems of database models”, in: Proc. 1st Symp. Math. Fund. of Database Syst., Springer, Berlin (1987), pp. 99–127.
4. P. Cohn, Universal Algebra, Harper and Row, New York (1965).
5. G. Burosch, J. Demetrovics, and G. O. H. Katona, “The poset of closures as a model of changing databases”, Order,4, 127–142 (1987).