1. Universality of data retrieval languages;Aho,1979
2. Program schemes with push-down stores;Brown;SIAM J. Comput.,1972
3. A relational model of data for large shared data banks;Codd;Comm. ACM,1970
4. Structure and complexity of relational queries;Chandra;J. Comput. System Sci.,1982
5. Algorithmic properties of structures;Engeler;Math. Systems Theory,1967