Abstract. The Description Logic E L has recently drawn considerable attention since, on the one hand, important inference problems such as the subsumption problem are polynomial. On the other hand, E L is used to define large biomedical ontologies. Unification in Description Logics has been proposed as a novel inference service that can, for example, be used to detect redundancies in ontologies. The main result of this paper is that unification in E L is decidable. More precisely, E L-unification is NP-complete, and thus has the same complexity as E L-matching. We also show that, w.r.t. the unification type, E L is less well-behaved: it is of type zero, which in particular implies that there are unification problems that have no finite complete set of unifiers.
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