1. School of Mathematics and Information Science; Guangzhou University; Guangzhou 510006 China
2. Key Laboratory of Mathematics and Interdisciplinary Sciences of Guangdong Higher Education Institutes; Guangzhou University; Guangzhou 510006 China
3. School of Economic and Management; Guangzhou University of Chinese Medicine; Guangzhou 51000 China
4. Department of Mathematics and Statistics; Curtin University of Technology; GPO Box U 1987 Perth 6845 WA Australia