Z. Gyöngyi, H. Garcia-Molina, and J. Pedersen. Proceedings of the Thirtieth international conference on Very large data bases - Volume 30, page 576--587. VLDB Endowment, (2004)
M. Noll, C. man Au Yeung, N. Gibbins, C. Meinel, and N. Shadbolt. Proceedings of the 32nd international ACM SIGIR conference on Research and development in information retrieval, page 612--619. New York, NY, USA, ACM, (2009)