Complexity of Subsumption in Extensions of EL

Abstract

During the last years, it has been shown that the description logic EL is well-suited for tractable reasoning. In particular, reasoning is even tractable w.r.t. general concept inclusion axioms, and various extensions of EL and their effects on the complexity of subsumption w.r.t. general concept inclusion axioms have been studied. In this thesis, we sharpen the border between tractability and intractability of subsumption in extensions of EL w.r.t. cyclic TBoxes. We provide two new extensions for which subsumption can be computed in polynomial time w.r.t. cyclic TBoxes. The first extends EL by role con- and disjunction in disjunctive normal form, primitive negation and p-admissible concrete domains, and the second by role con- and disjunction in disjunctive normal form and at-least restrictions. Moreover, we show that a combination of the two extensions leads to intractability of subsumption w.r.t. cyclic TBoxes, as well as EL extended by negation, disjunction, transitive closure over role names, functionality and concrete domains with abstract feature chains. This justifies the fact that–except for inverse roles which remain an open problem–both extensions are maximal in the sense that they cannot be further extended without losing tractability of subsumption w.r.t. cyclic TBoxes.