Constructions for supersaturation of eventown problems

Abstract

In this paper, we study the supersaturation problems of eventown. Given a family $\mathcal{A}$ of subsets of an $n$ element set, let op$(\mathcal{A})$ denote the number of distinct pairs $A,B\in \mathcal{A}$ for which $|A\cap B|$ is odd. We give extremal eventown constructions and show that for fixed $s\le 2^{\lfloor \frac{n}{2}\rfloor }-2$, there exists a collection of $2^{\lfloor \frac{n}{2}\rfloor }+s$ even-sized subsets of an $n$ element set that contains exactly $s\cdot 2^{\lfloor \frac{n}{2}\rfloor -1}$ pairwise intersections of odd size. This extends the range of $s$ in a conjecture proposed by O'Neill from $2^{\lfloor \frac{n}{2}\rfloor }-2^{\lfloor \frac{n}{4}\rfloor }$ to $2^{\lfloor \frac{n}{2}\rfloor }-2$. We also give a construction using symmetric designs to prove that when $k$ is even and $4k-1$ is a prime power, there exists a collection of $2^{\lfloor \frac{4k-1}{2}\rfloor }+s$ even-sized subsets of a $4k-1$ element set ${\mathcal{A}}_s$ with $op({\mathcal{A}}_s)=s\cdot 2^{\lfloor \frac{4k-1}{2}\rfloor -1}$, $1\le s\le 4k-1$.