We extend the description logic EL++ with reflexive roles
and range restrictions, and show that subsumption remains tractable if
a certain syntactic restriction is adopted. We also show that subsumption
becomes PSpace-hard (resp. undecidable) if this restriction is weakened
(resp. dropped). Additionally, we prove that tractability is lost when
symmetric roles are added: in this case, subsumption becomes ExpTime-
hard
D. Frankel, P. Hayes, E. Kendall, and D. McGuinness. 1st International Workshop on the Model-Driven Semantic Web (MDSW2004), Monterey, California, USA, (September 2004)
S. Grimm, B. Motik, and C. Preist. Proc. of the 3rd European Semantic Web Conference (ESWC 2006), volume 4011 of LNCS, page 575--589. Budva, Montenegro, Springer, (June 2006)