1. School of Mathematical Sciences, University of Southampton, Southampton, UK
2. Department of Mathematics and Applied Mathematics, Sefako Makgatho Health Sciences University, Pretoria, South Africa
3. Research & Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen, People's Republic of China
4. School of Mathematics and Statistics, Northwestern Polytechnical University, Xi'an, People's Republic of China
5. School of Mathematical Sciences, Zhejiang Normal University, Jinhua, People's Republic of China