1. Department of Mathematics, Yunnan Normal University, Kunming 650500, P.R. China.
2. College of Mathematics and Statistics, Xinyang Normal University, Xinyang 464000, P.R. China.
3. Department of Mathematics, Yunnan Normal University, Kunming 650500, P.R. China
4. Yunnan Key Laboratory of Modern Analytical Mathematics and Applications, Kunming, 650091, P.R. China.