In the Semantic Web

Ontologies play a central role in the development of the semantic web, as they provide precise definitions of shared terms in web resources. One important web ontology language is DAML+OIL; it has a formal semantics and a reasoning support through a mapping to the expressive description logic with the addition of inverse roles. In this paper, we present a probabilistic extension of , called P! , to allow for dealing with probabilistic ontologies in the semantic web. The description logic Pis based on the notion of probabilistic lexicographic entailment from probabilistic default reasoning. It allows to express rich probabilistic knowledge about concepts and instances, as well as default knowledge about concepts. We also present sound and complete reasoning techniques for P" , which are based on reductions to classical reasoning in # " ! and to linear programming, and which show in particular that reasoning in P# " ! is decidable. 1Dipartimento di Matematica e Informatica, Università degli Studi di Catania, Città Universitaria, Viale A. Doria 6, 95152 Catania, Italy; e-mail: [email protected]. 2Dipartimento di Informatica e Sistemistica, Università di Roma “La Sapienza”, Via Salaria 113, 00198 Rome, Italy; e-mail: [email protected]. Alternate address: Institut für Informationssysteme, Technische Universität Wien, Favoritenstraße 9-11, 1040 Vienna, Austria; e-mail: [email protected]. Acknowledgements: This work was partially supported by a Marie Curie Individual Fellowship of the European Community (Disclaimer: The authors are solely responsible for information communicated and the European Commission is not responsible for any views or results expressed) and by the Austrian Science Fund Project N Z29-INF. Copyright c $ 2002 by the authors 2 INFSYS RR 1843-02-06