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)
X. Wu, L. Zhang, and Y. Yu. WWW '06: Proceedings of the 15th international conference on World Wide Web, page 417--426. New York, NY, USA, ACM Press, (2006)