1. School of Computer Science and Engineering, Sun Yat-sen University, Guangzhou 510006, China. weixzh2010@163.com
2. School of Computer Science and Engineering, Sun Yat-sen Univ., Guangzhou 510006, China. issqdw@mail.sysu.edu.cn
3. School of Computer Science and Engineering, Sun Yat-sen University, Guangzhou 510006, China. caigya@mail2.sysu.edu.cn
4. Faculty of Informatics, Masaryk University, Brno, Czech Republic. gruska@fi.muni.cz
5. Instituto de Telecomunicações, Dept. de Matematica, Instituto Superior Técnico, Av. Rovisco Pais 1049-001 Lisbon, Portugal. pmat@math.ist.utl.pt