1. Yau Mathematical Sciences Center, Tsinghua University, Beijing, 100084, China.
2. Yau Mathematical Sciences Center, Tsinghua University, Beijing, 100084, China, and Yanqi Lake Beijing Institute of Mathematical Sciences and Applications, Beijing, 101408, China.
3. Department of Hydraulic Engineering, Tsinghua University, Beijing, 100084, China, and Breeding Base for State Key Laboratory of Land Degradation and Ecological Restoration in Northwest China, Ningxia University, Yinchuan, Ningxia, 750021, China.