1. Pawlak, Z.: Classification of objects by means of attributes. Polish Academy of Sciences, Technical Report PAS 429 (1981)
2. Pawlak, Z., Skowron, A.: Rudiments of rough sets. Information Sciences 177(1), 3–27 (2007)
3. Skowron, A., Stepaniuk, J.: Tolerance approximation spaces. Fundamenta Informaticae 27(2-3), 245–253 (1996)
4. Peters, J.F.: Near sets. Special theory about nearness of objects. Fundamenta Informaticae 75(1-4), 407–433 (2007)
5. Pavel, M.: Fundamentals of Pattern Recognition. Marcel Dekker, New York (1993)