1. School of Mathematics and Statistics, Lanzhou University, Lanzhou, Gansu, 730000, People’s Republic of China.
2. Corresponding author. School of Mathematics and Statistics, Lanzhou University, Lanzhou, Gansu, 730000, People’s Republic of China (corresponding address), and Center for Applied Mathematics in Gansu, Lanzhou, Gansu, 730000, People’s Republic of China.
3. Department of Mathematics, University of Miami, Coral Gables, FL 33146 USA.