1. Department of Mathematics, Zhejiang Normal University, Jinhua 321004, China
2. Department of Mathematics, Zhejiang University of Technology, Hangzhou 310032, China
3. Department of Mathematics, National Sun Yat-sen University, Kaohsiung 804, Taiwan
4. Center for Fundamental Science, Kaohsiung Medical University, Kaohsiung 80702, Taiwan
5. Department of Mathematics, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia