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In [ 1], the Acknowledgments section in the online version is incorrect.
The correct Acknowledgments section is: The authors Fuzhen Zhuang, Qing He, and Zhongzhi Shi are supported by the National Natural Science Foundation of China (No. 61175052, 60975039, 61203297, 60933004, 61035003) and the National High-tech R&D Program of China (863 Program) (No. 2012AA011003). A preliminary versionof this work has been published in ACM CIKM 2010 [ 2].
We apologize for the error.

For information on obtaining reprints of this article, please send e-mail to: tkde@computer.org.

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