T. Hofmann, J. Puzicha, and M. Jordan. Proceedings of the 1998 conference on Advances in neural information processing systems II, page 466--472. Cambridge, MA, USA, MIT Press, (1999)
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)