About sunflowers (1804.10050v2)

Abstract: Alon, Shpilka and Umans considered the following version of usual sunflower-free subset: a subset $\mbox{$\cal F$}\subseteq {1,\ldots ,D}^n$ for $D>2$ is sunflower-free if for every distinct triple $x,y,z\in \mbox{$\cal F$}$ there exists a coordinate $i$ where exactly two of $x_i,y_i,z_i$ are equal. Combining the polynomial method with character theory Naslund and Sawin proved that any sunflower-free set $\mbox{$\cal F$}\subseteq {1,\ldots ,D}^n$ has size $$ |\mbox{$\cal F$}|\leq c_D^n, $$ where $c_D=\frac{3}{2^{2/3}}(D-1)^{2/3}$. In this short note we give a new upper bound for the size of sunflower-free subsets of ${1,\ldots ,D}^n$. Our main result is a new upper bound for the size of sunflower-free $k$-uniform subsets. More precisely, let $k$ be an arbitrary integer. Let $\mbox{$\cal F$}$ be a sunflower-free $k$-uniform set system. Consider $M:=|\bigcup\limits_{F\in \mbox{$\cal F$}} F|. $ Then $$ |\mbox{$\cal F$}|\leq 3(\lceil\frac{2k}{3}\rceil+1)(2^{1/3}\cdot 3e)^k(\lceil\frac Mk\rceil -1)^{\lceil\frac{2k}{3}\rceil}. $$ In the proof we use Naslund and Sawin's result about sunflower-free subsets in ${1,\ldots ,D}^n$.