论文标题
交叉交叉家庭中的交叉点和不同的交叉点
Intersections and Distinct Intersections in Cross-intersecting Families
论文作者
论文摘要
令$ \ Mathcal {f},\ Mathcal {g} $为$ \ {1,2,\ ldots,n \} $的两个交叉截断家庭。令$ \ MATHCAL {F} \ WEDGE \ MATHCAL {G} $,$ \ MATHCAL {i}(\ MATHCAL {f},\ MATHCAL {G})$表示所有交叉口的家族$ f \ cap g $ at \ in \ in \ in \ Mathcal {交叉点$ f \ cap g $与$ f \ neq g,in \ mathcal {f},g \ in \ mathcal {g} $分别。对于固定的$ t \ subset \ {1,2,\ ldots,n \} $,让$ \ natercal {s} _t $是$ \ {1,2,\ ldots的所有$ k $ -subsets的家族,其中包含$ t $。在本文中,我们表明$ | \ MATHCAL {F} \ WEDGE \ MATHCAL {G} | $在$ \ Mathcal {f} = \ Mathcal {g} = \ Mathcal {s}当$ | \ Mathcal {i}(\ Mathcal {f},\ Mathcal {G})| $被最大化时,当$ \ Mathcal {f} = \ Mathcal {s} _ {\ Mathcal {s} _ {\ {\ {\ {1,2 \}}}}} \ cup \ cup \ Mathcal} \ Mathcal {s} _ {\ {1,4,5 \}} \ Cup \ Mathcal {s} _ {\ {\ {2,3,6 \}} $和$ \ Mathcal {G} = \ Mathcal {s} \ Mathcal {s} _ {\ {2,4 \}} \ Cup \ Mathcal {s} _ {\ {\ {1,4,4,6 \}} \ cup \ cup \ mathcal {s} _ {\ {\ {\ {\ {2,3,5 \}}}在$ t $ iSTERTING的家族中,最大数量的不同交叉点是针对$ n \ geq 3(t+2)^3k^2 $确定的。
Let $\mathcal{F},\mathcal{G}$ be two cross-intersecting families of $k$-subsets of $\{1,2,\ldots,n\}$. Let $\mathcal{F}\wedge \mathcal{G}$, $\mathcal{I}(\mathcal{F},\mathcal{G})$ denote the families of all intersections $F\cap G$ with $F\in \mathcal{F},G\in \mathcal{G}$, and all distinct intersections $F\cap G$ with $F\neq G, F\in \mathcal{F},G\in \mathcal{G}$, respectively. For a fixed $T\subset \{1,2,\ldots,n\}$, let $\mathcal{S}_T$ be the family of all $k$-subsets of $\{1,2,\ldots,n\}$ containing $T$. In the present paper, we show that $|\mathcal{F}\wedge \mathcal{G}|$ is maximized when $\mathcal{F}=\mathcal{G}=\mathcal{S}_{\{1\}}$ for $n\geq 2k^2+8k$, while surprisingly $|\mathcal{I}(\mathcal{F}, \mathcal{G})|$ is maximized when $\mathcal{F}=\mathcal{S}_{\{1,2\}}\cup \mathcal{S}_{\{3,4\}}\cup \mathcal{S}_{\{1,4,5\}}\cup \mathcal{S}_{\{2,3,6\}}$ and $\mathcal{G}=\mathcal{S}_{\{1,3\}}\cup \mathcal{S}_{\{2,4\}}\cup \mathcal{S}_{\{1,4,6\}}\cup \mathcal{S}_{\{2,3,5\}}$ for $n\geq 100k^2$. The maximum number of distinct intersections in a $t$-intersecting family is determined for $n\geq 3(t+2)^3k^2$ as well.