Affiliation: | 1. Center for Combinatorics, Nankai University, Tianjin 300071, PR China;2. School of Mathematics and Computer Science, Panzhihua University, Panzhihua 617000, PR China;3. Yangtze Center of Mathematics, Sichuan University, Chengdu 610064, PR China;4. Mathematical College, Sichuan University, Chengdu 610064, PR China |