Stefan Ellmauthaler

AAAI Conference 2022 Conference Paper

Answering Queries with Negation over Existential Rules

• Stefan Ellmauthaler
• Markus Krötzsch
• Stephan Mennicke

Ontology-based query answering with existential rules is well understood and implemented for positive queries, in particular conjunctive queries. For queries with negation, however, there is no agreed-upon semantics or standard implementation. This problem is unknown for simpler rule languages, such as Datalog, where it is intuitive and practical to evaluate negative queries over the least model. This fails for existential rules, which instead of a single least model have multiple universal models that may not lead to the same results for negative queries. We therefore propose universal core models as a basis for a meaningful (non-monotonic) semantics for queries with negation. Since cores are hard to compute, we identify syntactic conditions (on rules and queries) under which our core-based semantics can equivalently be obtained for other universal models, such as those produced by practical chase algorithms. Finally, we use our findings to propose a semantics for a broad class of existential rules with negation.

AIJ Journal 2018 Journal Article

Reactive multi-context systems: Heterogeneous reasoning in dynamic environments

• Gerhard Brewka
• Stefan Ellmauthaler
• Ricardo Gonçalves
• Matthias Knorr
• João Leite
• Jörg Pührer

Managed multi-context systems (mMCSs) allow for the integration of heterogeneous knowledge sources in a modular and very general way. They were, however, mainly designed for static scenarios and are therefore not well-suited for dynamic environments in which continuous reasoning over such heterogeneous knowledge with constantly arriving streams of data is necessary. In this paper, we introduce reactive multi-context systems (rMCSs), a framework for reactive reasoning in the presence of heterogeneous knowledge sources and data streams. We show that rMCSs are indeed well-suited for this purpose by illustrating how several typical problems arising in the context of stream reasoning can be handled using them, by showing how inconsistencies possibly occurring in the integration of multiple knowledge sources can be handled, and by arguing that the potential non-determinism of rMCSs can be avoided if needed using an alternative, more skeptical well-founded semantics instead with beneficial computational properties. We also investigate the computational complexity of various reasoning problems related to rMCSs. Finally, we discuss related work, and show that rMCSs do not only generalize mMCSs to dynamic settings, but also capture/extend relevant approaches w. r. t. dynamics in knowledge representation and stream reasoning.

FLAP Journal 2017 Journal Article

Abstract Dialectical Frameworks. An Overview.

• Gerhard Brewka
• Stefan Ellmauthaler
• Hannes Strass
• Johannes P. Wallner
• Stefan Woltran

JELIA Conference 2016 Conference Paper

Inconsistency Management in Reactive Multi-context Systems

• Gerhard Brewka
• Stefan Ellmauthaler
• Ricardo Gonçalves 0001
• Matthias Knorr 0001
• João Leite 0001
• Jörg Pührer

Abstract We address the problem of global inconsistency in reactive multi-context systems (rMCSs), a framework for reactive reasoning in the presence of heterogeneous knowledge sources that can deal with continuous input streams. Their semantics is given in terms of equilibria streams. The occurrence of inconsistencies, where rMCSs fail to have an equilibria stream, can render the entire system useless. We discuss various methods for handling this problem, following different strategies such as repairing the rMCS, or even relaxing the notion of equilibria stream so that it can go through inconsistent states.

ECAI Conference 2014 Conference Paper

Multi-Context Systems for Reactive Reasoning in Dynamic Environments

• Gerhard Brewka
• Stefan Ellmauthaler
• Jörg Pührer

We show in this paper how managed multi-context systems (mMCS) can be turned into a reactive formalism suitable for continuous reasoning in dynamic environments. We extend mMCS with (abstract) sensors and define the notion of a run of the extended systems. We then show how typical problems arising in online reasoning can be addressed: handling potentially inconsistent sensor input, modeling intelligent forms of forgetting, and controlling the reasoning effort spent by contexts. We also investigate the complexity of some important related decision problems.

IJCAI Conference 2013 Conference Paper

Abstract Dialectical Frameworks Revisited

• Gerhard Brewka
• Stefan Ellmauthaler
• Hannes Strass
• Johannes Peter Wallner
• Stefan Woltran

We present various new concepts and results related to abstract dialectical frameworks (ADFs), a powerful generalization of Dung’s argumentation frameworks (AFs). In particular, we show how the existing definitions of stable and preferred semantics which are restricted to the subcase of so-called bipolar ADFs can be improved and generalized to arbitrary frameworks. Furthermore, we introduce preference handling methods for ADFs, allowing for both reasoning with and about preferences. Finally, we present an implementation based on an encoding in answer set programming.