Colorings of $k$-sets with low discrepancy on small sets

Abstract

For $0<δ\leq 1$, let $R_k(m;δ)$ denote the smallest $N$ such that every coloring of $k$-element subsets by two colors yields an $m$-element set $M$ with relative discrepancy $δ$, which means that one color class has at least $(\frac{1+δ}2){m\choose k}$ elements. The number $R_k(m;δ)$ may be viewed as an extension of the usual $k$-hypergraph Ramsey number because $R_k(m)=R_k(m,1)$. Our main result is the following theorem. %\begin{theorem} For some constants $c,k_0$, and $\eps>0$, and for all $k\geq k_0$, $c\log k\leq n\leq k/11$, \[ R_k(k+n);2^{-\eps n})\geq \tw_{\lfloor k/n\rfloor}(2). \] %\end{theorem} In particular, for $n=\lceil c\log k\rceil$, we get a tower of height $δk/\log k$ and relative discrepancy polynomial in~$k$.