Random partition for Tokushige's $r$-wise intersecting conjecture

Abstract

Let $r\ge 3$ and let $1>p_1\ge p_2\ge \cdots \ge p_n>0$. Let $\hat{I}{¼}_{\mathbf{p}}$ denote the product measure on $2^{[n]}$ where each coordinate $i$ is included independently with probability $p_i$. A family $\mathcal{A}\subseteq 2^{[n]}$ is $r$-wise intersecting if $A_1\cap \cdots \cap A_r\ne \varnothing$ for all $A_1,\dots ,A_r\in \mathcal{A}$. In 2022, Tokushige proved that if $p_2<\frac{r-1}{r}$, then every $r$-wise intersecting family $\mathcal{A}\subseteq 2^{[n]}$ satisfies $\hat{I}{¼}_{\mathbf{p}}(\mathcal{A})\le p_1$, with equality only for stars centred at coordinates of maximum probability. He conjectured that the hypothesis $p_2<\frac{r-1}{r}$ can be replaced by $p_{r+1}<\frac{r-1}{r}$. In this paper, we prove this conjecture in full. The key novelty is the introduction of a new random partition method, which reduces the problem to at most $r$ coordinates and solves it exactly, thereby fully covering all cases with multiple supercritical coordinates.

10 pages