Cross-free families have linear size

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.

The main result of the paper was already proved by Knill in 1994. The current version of the manuscript is updated with this information