1. School of Mathematics and Statistics, Xuzhou University of Technology, Xuzhou 221008, Jiangsu, China
2. Department of Mathematics, Zhejiang Normal University, Jinhua 321004, Zhejiang, China
3. Department of Mathematics, King Abdulaziz University, Jeddah 21589, Saudi Arabia
4. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620-5700, USA
5. School of Mathematical and Statistical Sciences, North-West University, Mafikeng Campus, Private Bag X2046, Mmabatho 2735, South Africa