On the maximal size of $(a,b)$-town$\pmod k$ families

Abstract

A family $\mathcal{F}\subseteq\mathcal{P}(n)$ is an $(a,b)$-town$\pmod k$ if all sets in it have cardinality $a\pmod k$ and all pairwise intersections in it have cardinality $b\pmod k$. For $k=2$ the maximal size of such a family is known for each $a,b$, while for $k=3$ only $b-a\equiv 2 \pmod 3$ is fully understood. We provide a bound for $k=3$ when $b-a\equiv 1 \pmod 3$ and $n\equiv 2 \pmod 3$, which turns out to be tight for infinitely many such $n$. We also give sufficient conditions on the parameters $a,b,k,n$, which result in a better bound than the one from general settings by Ray-Chaudhuri--Wilson, in particular showing that this bound occurs infinitely often in a sense where all of $a,b,n$ can vary for a fixed $k$.