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Roman Schindlauer

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4 papers

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AIJ Journal 2008 Journal Article

Combining answer set programming with description logics for the Semantic Web

Thomas Eiter
Giovambattista Ianni
Thomas Lukasiewicz
Roman Schindlauer
Hans Tompits

We propose a combination of logic programming under the answer set semantics with the description logics SHIF ( D ) and SHOIN ( D ), which underly the Web ontology languages OWL Lite and OWL DL, respectively. To this end, we introduce description logic programs (or dl-programs), which consist of a description logic knowledge base L and a finite set P of description logic rules (or dl-rules). Such rules are similar to usual rules in nonmonotonic logic programs, but they may also contain queries to L, possibly under default negation, in their bodies. They allow for building rules on top of ontologies but also, to a limited extent, building ontologies on top of rules. We define a suite of semantics for various classes of dl-programs, which conservatively extend the standard semantics of the respective classes and coincide with it in absence of a description logic knowledge base. More concretely, we generalize positive, stratified, and arbitrary normal logic programs to dl-programs, and define a Herbrand model semantics for them. We show that they have similar properties as ordinary logic programs, and also provide fixpoint characterizations in terms of (iterated) consequence operators. For arbitrary dl-programs, we define answer sets by generalizing Gelfond and Lifschitz's notion of a transform, leading to a strong and a weak answer set semantics, which are based on reductions to the semantics of positive dl-programs and ordinary positive logic programs, respectively. We also show how the weak answer sets can be computed utilizing answer sets of ordinary normal logic programs. Furthermore, we show how some advanced reasoning tasks for the Semantic Web, including different forms of closed-world reasoning and default reasoning, as well as DL-safe rules, can be realized on top of dl-programs. Finally, we give a precise picture of the computational complexity of dl-programs, and we describe efficient algorithms and a prototype implementation of dl-programs which is available on the Web.

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IJCAI Conference 2005 Conference Paper

A Uniform Integration of Higher-Order Reasoning and External Evaluations in Answer-Set Programming

Thomas Eiter
Giovambattista Ianni
Roman Schindlauer
Hans

We introduce HEX programs, which are nonmonotonic logic programs admitting higher-order atoms as well as external atoms, and we extend the wellknown answer-set semantics to this class of programs. Higher-order features are widely acknowledged as useful for performing meta-reasoning, among other tasks. Furthermore, the possibility to exchange knowledge with external sources in a fully declarative framework such as Answer-Set Programming (ASP) is nowadays important, in particular in view of applications in the Semantic Web area. Through external atoms, HEX programs can model some important extensions to ASP, and are a useful KR tool for expressing various applications. Finally, complexity and implementation issues for a preliminary prototype are discussed.

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KR Conference 2004 Conference Paper

Combining Answer Set Programming with Description Logics for the Semantic Web

Thomas Eiter
Thomas Lukasiewicz
Roman Schindlauer
Hans Tompits

Towards the integration of rules and ontologies in the Semantic Web, we propose a combination of logic programming under the answer set semantics with the description logics SHIF(D) and SHOIN(D), which underly the Web ontology languages OWL Lite and OWL DL, respectively. This combination allows for building rules on top of ontologies but also, to a limited extent, building ontologies on top of rules. We introduce description logic programs (dl-programs), which consist of a description logic knowledge base L and a finite set of description logic rules (dl-rules) P. Such rules are similar to usual rules in logic programs with negation as failure, but may also contain queries to L, possibly default negated, in their bodies. We define Herbrand models for dl-programs, and show that satisfiable positive dl-programs have a unique least Herbrand model. More generally, consistent stratified dl-programs can be associated with a unique minimal Herbrand model that is characterized through iterative least Herbrand models. We then generalize the (unique) minimal Herbrand model semantics for positive and stratified dl-programs to a strong answer set semantics for all dl-programs, which is based on a reduction to the least model semantics of positive dl-programs. We also define a weak answer set semantics based on a reduction to the answer sets of ordinary logic programs. Strong answer sets are weak answer sets, and both properly generalize answer sets of ordinary normal logic programs. We then give fixpoint characterizations for the (unique) minimal Herbrand model semantics of positive and stratified dl-programs, and show how to compute these models by finite fixpoint iterations. Furthermore, we give a precise picture of the complexity of deciding strong and weak answer set existence for a dl-program.

LPAR Conference 2004 Conference Paper

Nonmonotonic Description Logic Programs: Implementation and Experiments

Thomas Eiter
Giovambattista Ianni
Roman Schindlauer
Hans Tompits

Abstract The coupling of description logic reasoning systems with other reasoning formalisms (possibly over the Web) is becoming an important research issue and calls for advanced methods and algorithms. Recently, several notions of description logic programs have been introduced, combining rule-based semantics with description logics. Among them are nonmonotonic description logic programs (or dl-programs for short) which combine nonmonotonic logic programs with description logics under a generalized version of the answer-set and the well-founded semantics, respectively, which are the predominant semantics for nonmonotonic logic programs. In this paper, we consider some technical issues regarding an efficient implementation for both semantics, which has been realized in a working prototype exploiting the two state-of-art tools DLV and RACER. A major issue in this respect is efficient interfacing between the two reasoning systems at hand, for which we devised special methods. Such methods may fruitfully be used for the implementation of systems of similar nature. Reported experimentation activities with our prototype show that the methods we have developed are effective and are a key for highly optimized nonmonotonic dl-program engines.

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