2 research outputs found
Query inseparability for ALC ontologies
We investigate the problem whether two ALC ontologies are indistinguishable (or inseparable) by means of queries in
a given signature, which is fundamental for ontology engineering tasks such as ontology versioning, modularisation,
update, and forgetting. We consider both knowledge base (KB) and TBox inseparability. For KBs, we give model-theoretic
criteria in terms of (finite partial) homomorphisms and products and prove that this problem is undecidable
for conjunctive queries (CQs), but 2ExpTime-complete for unions of CQs (UCQs). The same results hold if (U)CQs are
replaced by rooted (U)CQs, where every variable is connected to an answer variable. We also show that inseparability
by CQs is still undecidable if one KB is given in the lightweight DL EL and if no restrictions are imposed on the
signature of the CQs. We also consider the problem whether two ALC TBoxes give the same answers to any query
over any ABox in a given signature and show that, for CQs, this problem is undecidable, too. We then develop
model-theoretic criteria for HornALC TBoxes and show using tree automata that, in contrast, inseparability becomes
decidable and 2ExpTime-complete, even ExpTime-complete when restricted to (unions of) rooted CQs