1. Department of Mathematics and Statistics, University of North Carolina at Charlotte, Charlotte, NC 28223 USA.
2. Corresponding author. Department of Mathematics, National Center for Applied Mathematics Shenzhen, SUSTech International Center for Mathematics, Southern University of Science and Technology, Shenzhen 518055, People’s Republic of China.
3. Department of Mathematics, Southern University of Science and Technology, Shenzhen 518055, People’s Republic of China.