1. College of Science; China Three Gorges University; Yichang Hubei 443002 China
2. Wuhan Institute of Physics and Mathematics; Chinese Academy of Sciences; PO Box 71010 Wuhan 430071 China
3. Department of Mathematics and Computer Science; Jishou University; Jishou Hunan 416000 China
4. School of Mathematics and Statistics; Central South University; Changsha Hunan 410083 China