A default extension to description logics and its applications

Sammanfattning: This thesis discusses how to extend a family of knowledge representation formalisms known as description logics with default rules. Description logics are tailored to express knowledge in problem domains of a hierarchical or taxonomical nature, that is domains where the knowledge is easily expressed in terms of concepts, objects and relations. The proposed extension makes it possible to express "rules of thumb" in a restricted form of Reiter's default rules. We suggest that defaults of this form improve both the representational and inferential power of description logics. The default rules are used to compute the preferential instance relation which formally expresses when it is plausible that an object is an instance of a concept. We demonstrate the usefulness of the extension by describing two applications. The first solves a configuration problem where the goal is to find a suitable wine for a given meal where defaults are used as recommendations of wines. The second is a document retrieval application where default rules are used to enhance the search of WWW documents. The applications are based on an extended version of the knowledge-based system CLASSIC.