Sharp bounds on $k$-wise generalizations of oddtowns and eventowns

Abstract

For $\boldsymbolα = (α_1, \dots, α_k) \in \mathbb{F}_2^k$, an $\boldsymbolα$-town is a set family in which every $i$-wise intersection has parity $α_i$. Denote by $f_{\boldsymbolα}(n)$ the maximum size of an $\boldsymbolα$-town on $[n]$. The classical oddtown and eventown problems study the cases $\boldsymbolα = (1, 0)$ and $(0, 0)$, respectively. We determine the sharp asymptotics of $f_{\boldsymbolα}(n)$ for all $\boldsymbolα$, answering questions of Johnston--O'Neill and Wei--Zhang--Ge. We also study a symmetric variant $g_{\boldsymbolα}(n)$, in which $i$-wise intersection sizes $|F_1 \cap \dots \cap F_i|$ are replaced by $i$-wise intersection-union sizes $|F_1 \cap \dots \cap F_i| + |F_1 \cup \dots \cup F_i|$.