1. School of Information Science & Technology, East China Normal University, No. 500 Dongchuan Road, Shanghai 200241, China
2. Department of Computer and Information Science, University of Macau, Avenida Padre Tomas Pereira, Taipa 1356, Macau
3. Department of Mathematics, University of Salerno, Via Ponte Don Melillo, 84084 Fisciano, Italy