A two-center construction falsifies the Frankl-Wang product Hilton-Milner conjecture for n up to (c_ℓ-ε)k, while the conjecture holds with full extremal characterization for n>100ℓk^{2}.
A product version of the Hilton-Milner Theorem II
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abstract
Two families $\mathcal{F},\mathcal{G}$ of $k$-subsets of $\{1,2,\ldots,n\}$ are called {\it non-trivial cross-intersecting} if $F\cap G\neq \emptyset$ for all $F\in \mathcal{F}, G\in \mathcal{G}$ and $\cap \{F\colon F\in \mathcal{F}\}=\emptyset=\cap \{G\colon G\in\mathcal{G}\}$. In this note, we establish the product version of the Hilton-Milner Theorem for $k\geq 8$ in the full range. That is, if $\mathcal{F},\mathcal{G}\subset \binom{[n]}{k}$ are non-trivial cross-intersecting, $n\geq 2k+1$ and $k\geq 8$, then \[ |\mathcal{F}||\mathcal{G}|\leq \left(\binom{n-1}{k-1}- \binom{n-k-1}{k-1} +1\right)^2. \]
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math.CO 2years
2026 2representative citing papers
Proves |A|*|B| <= h(n,k)^2 for non-trivial cross-intersecting k-uniform families A, B on [n] when n >= 2k and k >= 3, with extremal pair characterization.
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On a conjecture regarding the product version of the Hilton-Milner theorem
A two-center construction falsifies the Frankl-Wang product Hilton-Milner conjecture for n up to (c_ℓ-ε)k, while the conjecture holds with full extremal characterization for n>100ℓk^{2}.
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A sharp product bound for non-trivial cross-intersecting families
Proves |A|*|B| <= h(n,k)^2 for non-trivial cross-intersecting k-uniform families A, B on [n] when n >= 2k and k >= 3, with extremal pair characterization.