Contributions to the theory of rough sets

We study properties of rough sets, that is, approximations to sets of records in a database or, more formally, to subsets of the universe of an information system. A rough set is a pair 〈L, U〉 such that L, U are definable in the information system and L ⊆ U. In the paper, we introduce a language, called the language of inclusion-exclusion, to describe incomplete specifications of (unknown) sets. We use rough sets in order to define a semantics for theories in the inclusion-exclusion language. We argue that our concept of a rough set is closely related to that introduced by Pawlak. We show that rough sets can be ordered by the knowledge ordering (denoted ≤_kn). We prove that Pawlak's rough sets are characterized as ≤_kn-greatest approximations. We show that for any consistent (that is, satisfiable) theory T in the language of inclusion-exclusion there exists a ≤_kn-greatest rough set approximating all sets X that satisfy T. For some classes of theories in the language of inclusion-exclusion, we provide algorithmic ways to find this best approximation. We also state a number of miscellaneous results and discuss some open problems.