1. Department of Aeronautics and Astronautics, Air Force Academy, Kaohsiung, Taiwan, R.O.C
2. Department of Mathematics, Zhejiang Normal University, Jinhua, People’s Republic of China
3. Department of Mathematics, King Abdulaziz University, Jeddah, Saudi Arabia
4. Department of Mathematics and Statistics, University of South Florida, Tampa, FL, USA
5. School of Mathematics, South China University of Technology, Guangzhou, People’s Republic of China