Colorings of $k$-sets with low discrepancy on small sets (2402.05286v1)

Abstract: According to Ramsey theorem, for every $k$ and $n$, if $N$ is sufficiently large, then for every 2-coloring $\psi$ of $k$-element subsets of $[N]$ there exists a monochromatic set $S\subseteq[N]$ (a set such that all $k$-element subsets of $S$ have the same color given by $\psi$), $|S|=m$. The least such number is denoted by $R_k(m)$. Old results of Erd\H os, Hajnal and Rado~\cite{erdos-hajnal-rado} imply that $R_k(m)\leq {\rm tw}{k}(c m)$, where ${\rm tw}_k(x)$ is the tower function defined by ${\rm tw}_1(x)=x$ and ${\rm tw}{i+1}(x)=2^{{\rm tw}i(x)}$. On the other hand, these authors also showed that if $N\leq {\rm tw}{k-1}(c'm^2)$, then there exists a coloring~$\psi$ such that there is no monochromatic $S\subseteq[N]$, $|S|=m$. We are interested in the question what more one can say when $N$ is smaller than ${\rm tw}{k-1}(m)$ and $m$ is only slightly larger than $k$. We will show that, for particular values of the parameters $k,m,N$, there are colorings such that on all subsets $S$, $|S|\geq m$, the number of $k$-subsets of one color is close to the number of $k$-subsets of the other color. In this abstract, for the sake of simplicity, we only state a special case of our main theorem. Theorem There exists $\epsilon>0$ and $k_0$ such that for every $k\geq k_0$, if $N< {\rm tw}{\lfloor\sqrt k\rfloor}(2)$, then there exists a coloring $\gamma:{[N]\choose k}\to{-1,1}$ such that for every $S\subseteq[N]$, $|S|\geq k+\sqrt k$, the following holds true [ \left|\sum{\gamma(X)\ |\ X\in\mbox{${S\choose k}$}}\right| \leq 2^{-\epsilon \sqrt k}\mbox{${|S|\choose k}$}. ]