1. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, China.
2. Corresponding author. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, China.
3. School of Mathematics and Statistics and Hubei Key Laboratory of Mathematical Sciences, Central China Normal University, Wuhan, China.
4. School of Mathematics and Statistics, Northwestern Polytechnical University, Xian, China.
5. Civil, Geological and Mining Engineering Department, Polytechnique Montreal, Montreal, Canada.
6. Department of Mathematics, The University of Hong Kong, Pokfulam, Hong Kong.