1. Department of Mathematics; Southeast University; Nanjing 210096 People's Republic of China
2. School of Mathematics and Physics; Jiangsu University of Science and Technology; Zhenjiang 212003 People's Republic of China
3. College of Mathematics and Statistics; Minnan Normal University; Zhangzhou 363000 People's Republic of China
4. Mathematics and Statistics; Louisiana Tech University; Ruston LA 71272 USA