1. School of Mathematics, Jilin University, Changchun 130021, China
2. Department of Industrial Electronics, School of Engineering, University of Minho, 4800-058 Guimares, Portugal
3. Key Laboratory of Symbol Computation and Knowledge Engineering of the Ministry of Education, College of Computer Science and Technology, Jilin University, 2699 Qianjin Street, Changchun 130012, China
4. School of Computer Science, Zhuhai College of Science and Technology, Zhuhai 519041, China