当前位置: X-MOL 学术Combinatorica › 论文详情
Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)
On Non-Empty Cross-Intersecting Families
Combinatorica ( IF 1.1 ) Pub Date : 2022-09-21 , DOI: 10.1007/s00493-021-4839-4
Chao Shi , Peter Frankl , Jianguo Qian

Let 2[n] and (\(\matrix{{\left[ n \right]} \cr i \cr } \)) be the power set and the collection of all i-subsets of {1, 2, …, n}, respectively. We call t (t ≥ 2) families \({{\cal A}_1},{{\cal A}_2}, \ldots ,{{\cal A}_t} \subseteq {2^{\left[ n \right]}}\) cross-intersecting if AiAj ≠ ∅ for any \({A_i} \in {{\cal A}_i}\) and \({A_j} \in {{\cal A}_j}\) with ij. We show that, for nk +l, lr ≥ 1, c > 0 and \({\cal A} \subseteq \left( {\matrix{{\left[ n \right]} \cr k \cr } } \right),{\cal B} \subseteq \left( {\matrix{{\left[ n \right]} \cr l \cr } } \right)\), if \({\cal A}\) and \({\cal B}\) are cross-intersecting and \(\left( {\matrix{{n - r} \cr {l - r} \cr } } \right) \le \left| {\cal B} \right| \le \left( {\matrix{{n - 1} \cr {l - 1} \cr } } \right)\), then

$$\left| {\cal A} \right| + c\left| {\cal B} \right| \le \max \left\{ {\left( {\matrix{n \cr k \cr } } \right) - \left( {\matrix{{n - r} \cr k \cr } } \right) + c\left( {\matrix{{n - r} \cr {l - r} \cr } } \right),\left( {\matrix{{n - 1} \cr {k - 1} \cr } } \right) + c\left( {\matrix{{n - 1} \cr {l - 1} \cr } } \right)} \right\}.$$

This implies a result of Tokushige and the second author (Theorem 3.1) and also yields that, for n ≥ 2k, if \({{\cal A}_1},{{\cal A}_2}, \ldots ,{{\cal A}_t} \subseteq \left( {\matrix{{\left[ n \right]} \cr k \cr } } \right)\) are non-empty cross-intersecting, then

$$\sum\limits_{i = 1}^t {\left| {{{\cal A}_i}} \right| \le \max \left\{ {\left( {\matrix{n \cr k \cr } } \right) - \left( {\matrix{{n - k} \cr k \cr } } \right) + t - 1,\,\,t\left( {\matrix{{n - 1} \cr {k - 1} \cr } } \right)} \right\},} $$

which generalizes the corresponding result of Hilton and Milner for t = 2. Moreover, the extremal families attaining the two upper bounds above are also characterized.

更新日期:2022-09-22
down
wechat
bug