1. Department of Mathematics, California State University LA, Los Angeles, CA 90032 USA.
2. Faculty of Computational Mathematics and Cybernetics, Shenzhen MSU-BIT University, International University Park Road 1, Shenzhen, China.
3. Department of Mathematical Sciences, Tsinghua University, Beijing, 100084, China; Institute of Mathematics, AMSS, Chinese Academy of Science, Beijing, 100190, China; and University of Chinese Academy of Science, Beijing, 100049, China.