1. Fakes, W. B. and Baeza-Yates, Information Retrieval.Data Structures and Algorithms(eds.), Prentice Hall, 1992.
2. Ho, T. B. and Funakoshi K., “Information retrieval using rough sets”, Journal of Japanese Society for Artificial Intelligence, Vol. 13, N. 3, 1998, 424–433.
3. Pawlak, Z., Rough sets: Theoretical Aspects of Reasoning about Data, Kluwer Academic Publishers, 1991.
4. Polkowski, L. and Skowron, A., Rough Sets in Knowledge Discovery 2. Applications, Case Studies and Software Systems(eds.), Physica-Verlag, 1998.
5. Skowron, A. and Stepaniuk, J., “Generalized approximation spaces”, The 3rd International Workshop on Rough Sets and Soft Computing, 1994, 156–163.