A few new oddtown and eventown problems

Abstract

Given a vector $\alpha = (\alpha_1, \ldots, \alpha_k) \in \mathbb{F}_2^k$, we say a collection of subsets $\mathcal{F}$ satisfies $\alpha$-intersection pattern modulo $2$ if all $i$-wise intersections consisting of $i$ distinct sets from $\mathcal{F}$ have size $\alpha_i \pmod{2}$. In this language, the classical oddtown and eventown problems correspond to vectors $\alpha=(1,0)$ and $\alpha=(0,0)$ respectively. In this paper, we determine the largest such set families of subsets on a $n$-element set with $\alpha$-intersection pattern modulo $2$ for all $\alpha \in \mathbb{F}_2^3$ and all $\alpha \in \mathbb{F}_2^4$ asymptotically. Lastly, we consider the corresponding problem with restrictions modulo $3$.