1. “Foundations of Databases”;Abitebeoul,1994
2. “Structural Complexity I”;Balcázar,1995
3. Computable Queries for Relational Data Bases;Chandra;Journal of Computer and System Sciences,1980
4. “Finite Model Theory”;Ebbinghaus,1999
5. Fagin R., Probabilities in Finite Models, in Notices of the American Mathematical Society, pág. A714, Abstract nro. 72T-E90, 1972.