Search

This chapter presents the theoretical basis of the vague knowledge search algorithmization of a rough method. It introduces some data granulation method which aggregates this data as rough sets of data or ways to search this data in the semantic networks. As a result of this method is the possibility of the rough sets description, analogically to sets in the classical theory of sets. We try to answer the question how the agent searching some knowledge can conceive the search of vague knowledge in the semantic networks: (1) if it can, accordingly to the semantic and the conceiving rules, describe the relationships between nodes in this semantic network which are identified as ways of searching knowledge, (2) if it can approximate sets of this ways by using ways described by some concepts and roles.

The approach to rough set theory proposed in this paper is based on the mutual correspondence of the concepts of extension and intension. It is different from the well-known approaches in the literature in that the upper approximations and the lower approximations of ‘unknown’ sets are considered as certain families of ‘known’ sets. This approach makes it possible to formulate necessary and sufficient conditions for the existence of operations on rough sets, which are analogous to classical operations on sets. The basic results presented in this paper, based on certain ideas of the second author, were formulated by the first author in his doctoral dissertation prepared under the supervision of the third author.

This paper presents a generalization of Pawlak’s conception of rough sets [6] and [7]. It is more general than Pawlak’s solution of the problem of the definability of sets, the knowledge of which is incomplete and vague. The authors’ conception is based on conception of contextual space [4], which was inspired by Ziarko’s approach [12] to rough sets. Rough sets introduced by Pawlak [6] are particular cases of contextual rough sets defined in the contextual approximation space. This space is defined axiomatically by means of so called context relations. Every contextual rough set determined by set X can be determined by the union of the lower approximation of X and a subset of the boundary of X. One of the important notions of the conception is the notion of an element of a contextual rough set which allows for formulating and proving the counterpart of the axiom of extensionality for contextual rough sets.

In the paper we present a formal description of rough sets within the limits of the generalized set theory, which is interpreted in the approximation of set theory. The rough sets are interpreted as an approximations, which are defined by means of the Pawlak’s rough sets.