An uniform version of Dvir and Moran's theorem (2105.04159v1)

Abstract: Dvir and Moran proved the following upper bound for the size of a family $\mbox{$\cal F$}$ of subsets of $[n]$ with $\mbox{Vdim}(\mbox{$\cal F$} \Delta \mbox{$\cal F$})\leq d$. Let $d\leq n$ be integers. Let $\mbox{$\cal F$}$ be a family of subsets of $[n]$ with $\mbox{Vdim}(\mbox{$\cal F$} \Delta \mbox{$\cal F$})\leq d$. Then [ \left|\mbox{$\cal F$}\right|\le 2\sum_{k=0}^{\lfloor d/2 \rfloor}\binom nk. ] Our main result is the following uniform version of Dvir and Moran's result. Let $d\leq n$ be integers. Let $\mbox{$\cal F$}$ be an uniform family of subsets of $[n]$ with $\mbox{Vdim}(\mbox{$\cal F$} \Delta \mbox{$\cal F$})\leq d$. Then [ \left|\mbox{$\cal F$}\right|\le 2 {n \choose \lfloor d/2 \rfloor}. ] Denote by $\mathbf{v_F}\in {0,1}^n$ the characteristic vector of a set $F \subseteq [n]$. Our proof is based on the following uniform version of Croot-Lev-Pach Lemma: Let $0\leq d\leq n$ be integers. Let $\mbox{$\cal H$}$ be a $k$-uniform family of subsets of $[n]$. Let $\mathbb F$ be a field. Suppose that there exists a polynomial $P(x_1, \ldots ,x_n,y_1, \ldots ,y_n)\in \mathbb F[x_1, \ldots ,x_n,y_1, \ldots ,y_n]$ with $\mbox{deg}(P)\leq d$ such that $P(\mathbf{v_F},\mathbf{v_F})\ne 0$ for each $F\in \mbox{$\cal H$}$ and $P(\mathbf{v_F},\mathbf{v_G})= 0$ for each $F\ne G\in \mbox{$\cal H$}$. Then [ \left|\mbox{$\cal H$}\right|\le 2 {n \choose \lfloor d/2 \rfloor}. ]