1. Department of Computer Science and Engineering; The Hong Kong University of Science and Technology; Clear Water Bay Kowloon Hong Kong
2. Institute of Algebra and Geometry; Faculty of Mathematics; Otto-von-Guericke University Magdeburg; Universitätsplatz 2 39106 Magdeburg Germany,
3. Department of Electrical and Electronic Engineering; South University of Science and Technology of China; Nanshan Shenzhen 518055 China,