Improved Bound on Sets Including No Sunflower with Three Petals

Abstract

A sunflower with $k$ petals, or $k$-sunflower, is a family of $k$ sets every two of which have a common intersection. Known since 1960, the sunflower conjecture states that a family ${\mathcal F}$ of sets each of cardinality $m$ includes a $k$-sunflower if $|{\mathcal F}| \ge c_k^m$ for some $c_k \in {\mathbb R}_{>0}$ depending only on $k$. The case $k=3$ of the conjecture was especially emphasized by Erd\"os, for which Kostochka's bound $c m! \left( \frac{\log \log \log m}{\log \log m} \right)^m$ on $|{\mathcal F}|$ without a 3-sunflower had been the best-known since 1997 until the recent development to update it to $c \log m$. This paper proves with an entirely different combinatorial approach that ${\mathcal F}$ includes three mutually disjoint sets if it satisfies the $\Gamma \left( c m^{\frac{1}{2}+ \delta} \right)$-condition for any given $\delta \in (0, 1/2)$. Here $c$ is a constant depending only on $\delta$, and the $\Gamma$-condition refers to \[ | \left\{ U~:~ U \in {\mathcal F} \textrm{~and~} S \subset U \right\}| < \left( c m^{\frac{1}{2}+ \delta} \right)^{-|S|} |{\mathcal F}|, \] for every nonempty set $S$. This poses an alternative proof of the 3-sunflower bound $\left( c m^{\frac{1}{2}+ \delta} \right)^m$.