A superlogarithmic saving for Oddtown modulo composite numbers

Abstract

Let $f_{\ell}(n)$ be the largest size of a family $\mathcal{A}\subseteq2^{[n]}$ such that no member has size divisible by $\ell$, while the intersection of every two distinct members has size divisible by $\ell$, and let $ω(\ell)$ denote the number of distinct prime divisors of $\ell$. For any prime power $\ell$, the classical answer is $f_{\ell}(n)=n$. When $ω(\ell)\geq 2$, Bukh, Chao, and Zheng recently proved $ω(\ell)n-O_{\ell}\left(n^{\frac{ω(\ell)-2}{ω(\ell)-1}}(\log n)^{C_{\ell}}\right)\leq f_{\ell}(n)\leqω(\ell)n-2ω(\ell)\log n+11$ for some $C_{\ell}>0$. When $\ell$ has at least two distinct odd prime divisors, they further used Fourier analysis to improve the upper bound to $f_{\ell}(n)\leqω(\ell)n-(2ω(\ell)+\varepsilon_{\ell})\log n$ for some $\varepsilon_{\ell}>0$, provided that $n$ is sufficiently large in terms of $\ell$ . For every fixed $\ell$ with $ω(\ell)\geq2$, we prove \[ f_{\ell}(n)\leqω(\ell)n-Ω_{\ell}(\log n\log\log n) \] for large $n$. The upper bound relies on a submatrix lemma of Bhowmick, Dvir, and Lovett, which is based on the bounded-torsion polynomial Freiman--Ruzsa conjecture recently proved by Gowers, Green, Manners, and Tao.