1. Garey, M.R., Johnson, D.S.: Computers and Intractability: A Guide to the Theory of NP-Completeness. Freeman, New York (1979)
2. Gutiérrez-Naranjo, M.A., Alonso-Jiménez, J.A., Borrego-Díaz, J.: A topological study of the upward refinement operators in ILP. In: ILP 2000 (2000) (work in progress track)
3. Gutiérez Naranjo, M.A.: Operadores de generalizaci´on para el aprendizaje clausal. Ph.D. Thesis. Dept. of Computer Science and AI. University of Seville (2002)
4. Lecture Notes in Computer Science;A. Hutchinson,1997
5. Laird, P.D.: Learning from Good and Bad Data. Kluwer Academic Publishers, Dordrecht (1988)