R. Agrawal, and R. Srikant. Proceedings of the 20th international conference on Very Large Data Bases (VLDB'94), page 478--499. Morgan Kaufmann, (September 1994)
J. Lafferty, A. McCallum, and F. Pereira. Proceedings of the Eighteenth International Conference on Machine Learning, page 282--289. San Francisco, CA, USA, Morgan Kaufmann Publishers Inc., (2001)