Maximal anti-Ramsey problems for posets

Abstract

We study the forbidden poset analog of the maximal anti-Ramsey problem introduced for graphs by Burr, Erd\H os, Graham, and Sós. For integers $m\le 2^n$ and poset $P=(P,\preceq)$, we introduce $\mathrm{ar_m}(n,m,P)$ (and $\mathrm{ar^*_m}(n,m,P)$) to denote the minimum integer $k$ such that there exists a family $\mathcal{F}\subseteq 2^{[n]}$ with $|\mathcal{F}|=m$ and a coloring $\mathcal{F}\rightarrow [k]$ with all weak (strong) copies of $P$ being rainbow. As long as there exist $P$-free families of size $m$, these parameters equal 1. It is known that the largest size $La(n,P)$ ($La^*(n,P)$) of weak (strong) $P$-free families has order of magnitude $Θ(\binom{n}{\lfloor n/2\rfloor})$ unless $P$ is the antichain $A_k$ on $k$ elements. In this paper we study $\mathrm{ar_m}(n,m,P)$ and $\mathrm{ar^*_m}(n,m,P)$ in two regimes of $m$. We determine the asymptotics of these parameters for all posets $P$ when $m=2^n$. We also consider the case $m=Θ(\binom{n}{\lfloor n/2\rfloor})$. It is shown that for any connected poset $P$ and integer $k$, there exist integers $m_{P,k}$ and $m^*_{P,k}$ such that to color the middle $k$ layers of the Boolean lattice with all weak or strong copies of $P$ being rainbow, one needs $Θ(n^{m_{P,k}})$ or $Θ(n^{m^*_{P,k}})$ colors. For tree posets $T$, one has $m_{T,k}=m^*_{T,k}$. We conjecture that for any tree poset $T$, and positive real $\varepsilon$, $\mathrm{ar_m}(n,m,T),\mathrm{ar^*_m}(n,m,T)=Ω(n^{m_{T,k}})$ holds provided $m\ge (k-1+\varepsilon)\binom{n}{\lfloor n/2\rfloor}$. We prove our conjecture on $\mathrm{ar_m}(n,m,T)$ for an infinite class of tree posets.