Sebastian Hellmann Jens Lehmann Sören Auer Learning of OWL Class Descriptions on Very Large Knowledge Bases. 25-48 2009 5 Int. J. Semantic Web Inf. Syst. 2 http://www.igi-global.com/articles/details.asp?ID=35036 db/journals/ijswis/ijswis5.html#HellmannLA09