H. Ma, H. Yang, I. King, and M. Lyu. CIKM '08: Proceeding of the 17th ACM conference on Information and knowledge mining, page 709--718. New York, NY, USA, ACM, (2008)
T. Hofmann. volume 2003 of Special issue of the SIGIR forum, New York, NY, International Conference on Research and Development in Information Retrieval, ACM Press, (2003)