Fabio Papacchini

Dichotomies in Ontology-Mediated Querying with the Guarded Fragment

We study the complexity of ontology-mediated querying when ontologies are formulated in the guarded fragment of first-order logic (GF). Our general aim is to classify the data complexity on the level of ontologies where query evaluation w.r.t. an ontology O is considered to be in PTime if all (unions of conjunctive) queries can be evaluated in PTime w.r.t. O and coNP-hard if at least one query is coNP-hard w.r.t. O. We identify several large and relevant fragments of GF that enjoy a dichotomy between PTime and coNP, some of them additionally admitting a form of counting. In fact, almost all ontologies in the BioPortal repository fall into these fragments or can easily be rewritten to do so. We then establish a variation of Ladners Theorem on the existence of NP-intermediate problems and use this result to show that for other fragments, there is provably no such dichotomy. Again for other fragments (such as full GF), establishing a dichotomy implies the Feder-Vardi conjecture on the complexity of constraint satisfaction problems. We also link these results to Datalog-rewritability and study the decidability of whether a given ontology enjoys PTime query evaluation, presenting both positive and negative results.

Terminating Minimal Model Generation Procedures for Propositional Modal Logics

Model generation and minimal model generation are useful for tasks such as model checking and for debugging of logical specifications. This paper presents terminating procedures for the generation of models minimal modulo subset-simulation for the modal logic K and all combinations of extensions with the axioms T, B, D, 4 and 5. Our procedures are minimal model sound and complete. Compared with other minimal model generation procedures, they are designed to have smaller search space and return fewer models. In order to make the models more effective for users, our minimal model criterion is aimed to be semantically meaningful, intuitive and contain a minimal amount of information. Depending on the logic, termination is ensured by a variation of equality blocking.

Computing minimal models modulo subset-simulation for modal logics

In this paper we propose a novel minimality criterion for models of modal logics based on a variation of the notion of simulation, called subset-simulation. We present a minimal model sound and complete tableau calculus for the generation of this new kind of minimal models for the multi-modal logic K (m), and we discuss extensions to cover more expressive logics. The generation of minimal models is performed incrementally by using a minimality test to close branches representing non-minimal models, or to update the set of minimal models. Subset-simulation minimal models have the advantage that they are semantically more natural than models obtained by using syntactic minimality criteria.

Horn-Rewritability vs PTime Query Evaluation in Ontology-Mediated Querying

© 2018 International Joint Conferences on Artificial Intelligence. All right reserved. In ontology-mediated querying with an expressive description logic (DL) L, two desirable properties of a TBox T are (1) being able to replace T with a TBox formulated in the Horn-fragment of L without affecting the answers to conjunctive queries (CQs) and (2) that every CQ can be evaluated in PTIME w.r.t. T . We investigate in which cases (1) and (2) are equivalent, finding that the answer depends on whether the unique name assumption (UNA) is made, on the DL under consideration, and on the nesting depth of quantifiers in the TBox. We also clarify the relation between query evaluation with and without UNA and consider natural variations of property (1).