An Erdős--Trotter problem on antichains with multiplicity $r$ on each occurring level

Abstract

Fix an integer $r\ge2$. For each $n$ we consider families $\mathcal F\subseteq 2^{[n]}$ that form an antichain and have the property that, for every $t$, if there exists $A\in\mathcal F$ with $|A|=t$ then there exist at least $r$ members of $\mathcal F$ of size $t$. A problem of Erdős and Trotter asserts that, for each fixed $r$, there exists a threshold $n_0(r)$ such that whenever $n>n_0(r)$ one can achieve $n-3$ distinct set sizes in such a family, and asks for estimates on $n_0(r)$. We compute that $n_0(2)=3$ and $n_0(3)=8$. For all $r\ge4$ we prove matching linear bounds up to lower-order terms, namely $$ 2r+2 \le n_0(r) \le 2r+2\log_2 r + O(\log_2\log_2 r). $$ In particular, $n_0(r) = 2r + o(r)$.