Both OWL-DL and function-free Horn rules are decidable fragments of first-order logic with interesting, yet orthogonal expressive power. A combination of OWL-DL and rules is desirable for the Semantic Web; however, it might easily lead to the undecidability of interesting reasoning problems. Here, we present a decidable such combination where rules are required to be DL-safe: each variable in the rule is required to occur in a non-DL-atom in the rule body. We discuss the expressive power of such a combination and present an algorithm for query answering in the related logic SHIQ extended with DL-safe rules, based on a reduction to disjunctive programs.

Abstract. Millions of research funding has been put down to develop- what I call- old forms- of reasoning that are characterized by strong focus on theoretical properties and strict adherence to the completeness properties of reasoning procedures. Despite the large amount of work and results that have been achieved, various benchmarks reinstate that the progress does not su ce for needs in the scale of many enterprise applications and the Web. We believe that a fundamental change in research on reasoning is required, giving up basic assumptions such as completeness to gain performance required in many real-world applications. These new paths should start with a deriving a clear understanding of what types of questions should be solved, where reasoning can help, where properties like soundness and completeness are really required and what impact of departing from those properties is acceptable. The standard ontology languages that have been standardized by W3C recently have varying but high complexity ranging from PTIME (RDFS) to NEX-

Deliverable D2.1.2 (WP2.1) This deliverable shows examples about approximating symbolic inference engines in a Semantic Web environment. Approaches of language weakening, knowledge compilation, and approximated deduction are presented. The last one is evaluated in practical applications with mixed results.

interoperability of ontology development tools using OWL as interchange language Raúl García-Castro (UPM) with contributions from:

Defining a benchmark suite for evaluating the import of OWL Lite ontologies