L. Wu, L. Yang, N. Yu, and X. Hua. WWW '09: Proceedings of the 18th international conference on World wide web, page 361--370. New York, NY, USA, ACM, (2009)
S. Bao, G. Xue, X. Wu, Y. Yu, B. Fei, and Z. Su. WWW '07: Proceedings of the 16th international conference on World Wide Web, New York, NY, USA, ACM Press, (2007)