1. School of Mathematics and Information Science; Guangzhou University; Guangzhou 510006 China
2. Key Laboratory of Mathematics and Interdisciplinary Sciences, Guangdong Higher Education Institutes; Guangzhou University; Guangzhou 510006 China
3. Faculty of Science and Technology; University of Macau; Macau 3001 China
4. Department of Mathematics and Statistics; Curtin University of Technology; GPO Box U 1987 Perth, Western Australia 6845 Australia