1. Z. Pawlak, Rough Sets, Theoretical Aspects of Reasoning about Data, Dordrecht: Kluwer (1991).
2. M. Banerjee and M. K. Chakraborty, Rough Algebra, Ics Research Report, Institute of Computer Science, Warsaw University of Technology,47/93.
3. Q. Liu, The Resolution for Rough Prepositional Logic with Lower (L) and Upper (H) Approximate Operators,LNAI 1711, Springer,ll,(1999), 352–356.
4. Q. Liu, Operator Rough Logic and Its Resolution Principles, Journal of Computer 5, (1998), 476–480,(in Chinese).
5. A. Nakamura, Graded Modalities in Rough Logic, Rough Sets in Knowledge Discovery I, by L. Polkaowski and A. Skowron, Phsica-verlag,Heidelberg, (1998).