1. Department of Mathematics, Zhejiang Normal University, Jinhua 321004, People's Republic of China
2. College of Mathematics and Systems Science, Shandong University of Science and Technology, Qingdao 266590, Shandong, People's Republic of China
3. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620, USA
4. Department of Mathematical Sciences, North-West University, Mmabatho 2735, South Africa