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arXiv · 2603.05443

Cross-free families have linear size

István Tomon
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Abstract

Two subsets $A$ and $B$ of a ground set $X$ are crossing if none of the four sets $A\setminus B,B\setminus A,A\cap B, X\setminus (A\cup B)$ are empty. Almost fifty years ago, Karzanov and Lomonosov conjectured that every family of subsets of an $n$-element ground set with no $k$-pairwise crossing members has size $O(kn)$. We prove the bound $O_k(n)$, settling (arguably) the main problem about the growth rate of such families. In fact, we prove the stronger statement that every $k$-wise laminar family has at most linear size. Here a family is $k$-wise laminar if it contains no antichain of size $k$ whose members have nonempty common intersection.

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István Tomon. 2026-03-05. Cross-free families have linear size. https://arxiv.org/abs/2603.05443

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