1. Department of Mathematics, Zhejiang Normal University, Jinhua, 321004 Zhejiang, China
2. Department of Mathematics, King Abdulaziz University, Jeddah 21589, Saudi Arabia
3. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620-5700, USA
4. Material Science Innovation and Modelling, Department of Mathematical Sciences, North-West University, Mafikeng Campus, Mmabatho 2735, South Africa