Affiliation: | 1. Department of Applied Mathematics, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, P. R. China;2. Center for Combinatorics, Nankai University, Tianjin 300071, P. R. China;3. Xi'an-Budapest Joint Research Center for Combinatorics, Northwestern Polytechnical University, Xi'an, Shaanxi 710129, P. R. China |