1. College of Applied Mathematics, Chengdu University of Information Technology, Chengdu Sichuan 610225, P. R. China
2. School of Mathematical Sciences, Sichuan Normal University, Chengdu, Sichuan 61068, P. R. China
3. Division of Computer Engineering, Hoseo University, Asan 31499, Republic of Korea
4. School of Mathematics and Physics, Southwest University of Science and Technology, Mianyang 621010, P. R. China