The realization of the Semantic Web vision, in which computational logic has a prominent role, has stimulated a lot of research on combining rules and ontologies, which are formulated in different formalisms, into a framework that is more useful for describing semantic content. In particular, combining logic programming with the Web Ontology Language (OWL), which is a standard based on description logics, emerged as an important issue for linking the Rules and Ontology Layers of the Semantic Web. Non-monotonic description logic programs (or dl-programs) were introduced for such a combination, in which a pair (L,P) of a description logic knowledge base L and a set of rules P with negation as failure is given a model-based semantics that generalizes the answer set semantics of logic programs. In this paper, we reconsider dl-programs and present a well-founded semantics for them as an analog for the other main semantics of logic programs. It generalizes the canonical definition of the well-founded semantics based on unfounded sets, and, as we show, lifts many of the well-known properties from ordinary logic programs to dl-programs. Among these properties: our semantics amounts to a partial model approximating the answer set semantics, which yields for positive and stratified dl-programs a total model coinciding with the answer set semantics; it has polynomial data complexity provided the access to the description logic

We introduce ELP as a decidable fragment of the Semantic Web Rule Language (SWRL) that admits reasoning in polynomial time. ELP is based on the tractable description logic EL ++, and encompasses an extended notion of the recently proposed DL rules for that logic. Thus ELP extendsEL ++ with a number of features introduced by the forthcoming OWL 2, such as disjoint roles, local reflexivity, certain range restrictions, and the universal role. We present a reasoning algorithm based on a translation of ELP to Datalog, and this translation also enables the seamless integration of DL-safe rules into ELP. While reasoning with DL-safe rules as such is already highly intractable, we show that DL-safe rules based on the Description Logic Programming (DLP) fragment of OWL 2 can be admitted in ELP without losing tractability.

We propose a description-logic style extension of OWL 2 with nominal schemas which can be used like “variable nominal classes”within axioms. This feature allows ontology languages to express arbitrary DL-safe rules (as expressible in SWRL or RIF) in their native syntax. We show that adding nominal schemas to OWL 2 does not increase the worst-case reasoning complexity, and we identify a novel tractable language SROELV 3(⊓, ×) that is versatile enough to capture the lightweight languages OWL EL and OWL RL.

Open Answer Set Programming (OASP) is an attractive framework for integrating ontologies and rules. Although several decidable fragments of OASP have been identified, few reasoning procedures exist. In this paper, we provide a sound, complete, and terminating algorithm for satisfiability checking w.r.t. forest logic programs, a fragment where rules have a tree shape and allow for inequality atoms and constants. We further introduce f-hybrid knowledge bases, a hybrid framework where SHOQ knowledge bases and forest logic programs co-exist, and we show that reasoning with such knowledge bases can be reduced to reasoning with forest logic programs only. We note that f-hybrid knowledge bases do not require the usual (weakly) DL-safety of the rule component, providing thus a genuine alternative approach to hybrid reasoning.

Abstract. Ontological metamodeling has a variety of applications yet only very restricted forms are supported by OWL 2 directly. We propose a novel encoding scheme enabling class-based metamodeling inside the domain ontology with full reasoning support through standard OWL 2 reasoning systems. We demonstrate the usefulness of our method by applying it to the OntoClean methodology. En passant, we address performance problems arising from the inconsistency diagnosis strategy originally proposed for OntoClean by introducing an alternative technique where sources of conflicts are indicated by means of marker predicates. 1

Abstract. Open Answer Set Programming (OASP) is a knowledge representation paradigm that allows for a tight integration of Logic Programming rules and Description Logic ontologies. Although several decidable fragments of OASP exist, no reasoning procedures for such expressive fragments were identified so far. We provide an algorithm that checks satisfiability in nexptime for the fragment of exptime-complete simple conceptual logic programs. 1

Abstract. The CEL system is known for its scalability of reasoning in the lightweight DL EL ++ which has been proved suitable for several ontology applications, most notably from the life science domain. Recently, the DL EL ++ has been adopted as the logical underpinning of the OWL 2 EL profile of the new Web Ontology Language which potentially attracts new folks of CEL’s users. To seamlessly integrate the reasoner to the OWL user community, we have implemented the OWL API for CEL. This paper describes the challenges, design decision and architecture of this implementation. Additionally, we present experimental results which highlight the scalability of the reasoner, as well as demonstrate a low overhead of our OWL API implementation. 1

Abstract. Description Logic Programs (DLP) have been described as a description logic (DL) that is in the “expressive intersection ” of DL and datalog. This is a very weak guideline for defining DLP in a way that can be claimed to be optimal or maximal in any sense. Moreover, other DL fragments such asEL and Horn-SHIQ have also been “expressed ” using datalog. Is DLP just one out of many equal DLs in this “expressive intersection”? This paper attempts to clarify these issues by characterising DLP with various design principles that clearly distinguish it from other approaches. A consequent application of the introduced principles leads to the definition of a significantly larger variant of DLP which we conjecture to be maximal in a concrete sense. A preliminary report on the proof of this maximality is provided. While DLP is used as a concrete (and remarkably complex) example in this paper, we argue that similar approaches can be applied to find canonical definitions for other fragments of logical languages. 1

Abstract. With OWL 2 about to be completed, it is the right time to start discussions on possible future modifications of OWL. We present here a number of suggestions in order to discuss them with the OWL user community. They encompass expressive extensions on polynomial OWL 2 profiles, a suggestion for an OWL Rules language, and expressive extensions for OWL DL.

Abstract. The Family History Knowledge Base (FHKB) was presented at OWLED in 2008. The FHKB uses a rich object property hierarchy, including many OWL 2 features, to derive many entailments on some 450 individuals representing the Stevens family. In the ABox, only parent relationships and some sibling relationships (either isSisterOf or isBrotherOf) are asserted. Using a sparse assertion of brother or sister relationships, together with information about gender, other sibling relationships should be able to be inferred. The inability to do this in OWL has been described as the ‘Man-Man ’ problem, and various workarounds have been discussed. We describe a new solution to this issue, implemented in the reasoner FaCT++. This solution allows to capture axioms such as ‘My male siblings are my brothers’, and we have added them to the FHKB. The number of entailments about, for instance, sibling relationships increases significantly without increasing the number of asserted facts about members of the Stevens family. 1