J. Tang, J. Zhang, L. Yao, J. Li, L. Zhang, und Z. Su. Proceeding of the 14th ACM SIGKDD international conference on Knowledge discovery and data mining, Seite 990--998. New York, NY, USA, ACM, (2008)
L. Zhang, J. Barnden, R. Hendley, und A. Wallington. Technologies for E-Learning and Digital Entertainment, Volume 3942 von Lecture Notes in Computer Science, Springer Berlin / Heidelberg, (2006)