Intelligent Data Center and Department of Mathematics, Sun Yat-Sen University, Xingang Road West, 510275 Guangzhou, China. stsddq@mail.sysu.edu.cn.|Department of Library and Information Science, Yonsei University, 50 Yonsei-ro, Seoul 120-749, Korea. min.song@yonsei.ac.kr.|Department of Library and Information Science, Yonsei University, 50 Yonsei-ro, Seoul 120-749, Korea. min.song@yonsei.ac.kr.