4 research outputs found
Checking Chase Termination over Ontologies of Existential Rules with Equality
The chase is a sound and complete algorithm for conjunctive query answering
over ontologies of existential rules with equality. To enable its effective
use, we can apply acyclicity notions; that is, sufficient conditions that
guarantee chase termination. Unfortunately, most of these notions have only
been defined for existential rule sets without equality. A proposed solution to
circumvent this issue is to treat equality as an ordinary predicate with an
explicit axiomatisation. We empirically show that this solution is not
efficient in practice and propose an alternative approach. More precisely, we
show that, if the chase terminates for any equality axiomatisation of an
ontology, then it terminates for the original ontology (which may contain
equality). Therefore, one can apply existing acyclicity notions to check chase
termination over an axiomatisation of an ontology and then use the original
ontology for reasoning. We show that, in practice, doing so results in a more
efficient reasoning procedure. Furthermore, we present equality model-faithful
acyclicity, a general acyclicity notion that can be directly applied to
ontologies with equality
Pseudo-contractions as Gentle Repairs
Updating a knowledge base to remove an unwanted consequence is a challenging task. Some of the original sentences must be either deleted or weakened in such a way that the sentence to be removed is no longer entailed by the resulting set. On the other hand, it is desirable that the existing knowledge be preserved as much as possible, minimising the loss of information. Several approaches to this problem can be found in the literature. In particular, when the knowledge is represented by an ontology, two different families of frameworks have been developed in the literature in the past decades with numerous ideas in common but with little interaction between the communities: applications of AGM-like Belief Change and justification-based Ontology Repair. In this paper, we investigate the relationship between pseudo-contraction operations and gentle repairs. Both aim to avoid the complete deletion of sentences when replacing them with weaker versions is enough to prevent the entailment of the unwanted formula. We show the correspondence between concepts on both sides and investigate under which conditions they are equivalent. Furthermore, we propose a unified notation for the two approaches, which might contribute to the integration of the two areas
Usage Policies for Decentralised Information Processing
Owners impose usage restrictions on their information, which can be based e.g. on privacy laws, copyright law or social conventions. Often, information is processed in complex constellations without central control. In this work, we introduce technologies to formally express usage restrictions in a machine-interpretable way as so-called policies that enable the creation of decentralised systems that provide, consume and process distributed information in compliance with their usage restrictions