1. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620, USA
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 Mathematics, South China University of Technology, Guangzhou 510640, China
6. School of Sciences, North-West University, Mafikeng Campus, Mmabatho 2735, South Africa