Affiliation: | 1. School of Mathematics and Statistics, Northeast Normal University, Changchun, Jilin 130024, PR China;2. College of Mathematics, Jilin University, Changchun, Jilin 130012, PR China;3. Chern Institute of Mathematics and LPMC, Nankai University, Tianjin 300071, PR China;4. Department of Mathematics and Statistics, Utah State University, Logan, Utah 84322, USA |