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, USA
4. School of Mathematical and Statistical Sciences, North-West University, Mafikeng Camus, Private Bag X2046, Mmabatho 2735, South Africa
5. Department of Mathematics, Florida A& M University, Tallahassee, FL 32307, USA
6. College of Art and Sciences, Shanghai Maritime University, Shanghai 201306, China