The basic idea of the combined approach to query answering in the presence of ontologies is to materialize the consequences of the ontology in the data and then use a limited form of query rewriting to deal with infinite materializations. While this approach is efficient and scalable for ontologies that are formulated in the basic version of the description logic DL-Lite, it incurs an exponential blowup during query rewriting when DL-Lite is extended with the popular role hierarchies. In this paper, we show how to replace the query rewriting with a filtering technique. This is natural from an implementation perspective and allows us to handle role hierarchies without an exponential blowup. We also carry out an experimental evaluation that demonstrates the scalability of this approach.

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Current standards on agent communication languages explain communication by changes in the mental states of agents. It is rather a striking issue that none of the popular agent middleware adheres to this semantics in a formal way. Therefore, this paper tries to close the gap between agent communication theory and practice by implementing a logic that is capable of representing agents' beliefs and intentions. An interesting property of this language, named ALC BI , is that it is a hybrid of modal and description logics. Due to its description logic foundation, it can also be used as an ontology definition language. Thus, it is well suited for agents working on the semantic web. In this paper, a sound and complete decision algorithm as well as other reasoning services for the logic are presented. Finally, the application of the language in a multi-agent development framework is discussed.

The Beth definability property, a well-known property from classical logic, is investigated in the context of description logics: if a general L-TBox implicitly defines an L-concept in terms of a given signature, where L is a description logic, then does there always exist over this signature an explicit definition in L for the concept? This property has been studied before and used to optimize reasoning in description logics. In this paper a complete classification of Beth definability is provided for extensions of the basic description logic ALC with transitive roles, inverse roles, role hierarchies, and/or functionality restrictions, both on arbitrary and on finite structures. Moreover, we present a tableau-based algorithm which computes explicit definitions of at most double exponential size. This algorithm is optimal because it is also shown that the smallest explicit definition of an implicitly defined concept may be double exponentially long in the size of the input TBox. Finally, if explicit definitions are allowed to be expressed in first-order logic, then we show how to compute them in single exponential time.