1. Corresponding author. Department of Mathematics, Northwest Normal University, Lanzhou 730070, People’s Republic of China (Corresponding address), and Gansu Provincial Research Center for Basic Disciplines of Mathematics and Statistics, Lanzhou 730070, People’s Republic of China.
2. School of Mathematical Sciences, Guizhou Normal University, Guiyang 550001, People’s Republic of China.
3. Department of Mathematics, Northwest Normal University, Lanzhou 730070, People’s Republic of China.