1. School of Mathematical Sciences and Physics, North China Electric Power University, Beijing 102206, China
2. Department of Mathematics, Zhejiang Normal University, Jinhua 321004, Zhejiang, China
3. Department of Mathematics and Statistics, University of South Florida, Tampa FL 33620, USA
4. Department of Mathematical Sciences, North-West University, Mafikeng Campus, Private Bag X 2046, Mmabatho 2735, South Africa
5. College of Mathematics and System Sciences, Shandong University of Science and Technology, Qingdao 266590, Shandong, China
6. Department of Mathematics, King Abdulaziz University, Jeddah, Saudi Arabia
7. School of Mathematical Sciences, University of Science and Technology of China, Hefei 230026, Anhui, China