Cube Height, Cube Width and Related Extremal Problems for Posets

TL;DR

We study inclusion representations of finite posets through cube height $ch(P)$ and cube width $cw(P)$, proving $cw(P)\le |P|$ and introducing the irreducible-inclusion-representation parameter $iir(P)$. We characterize the case $iir(P)=|P|$ by three structural properties and develop decomposition tools for components and blocks to relate $ch$, $\dim_2$, $cw$, and $iir$ under poset operations. The paper further provides polynomial-time criteria for when $\dim_2(P)=|P|$ or $cw(P)=|P|$, with precise classifications for the equalities and a discussion of the deeper structure of posets in the maximal-ir representations class. The results connect these extremal questions to induced-saturation problems and suggest rich structural phenomena and future directions in the study of poset representations.

Abstract

Given a poset $P$, a family $\mathcal{S}=\{S_x:x\in P\}$ of sets indexed by the elements of $P$ is called an inclusion representation of $P$ if $x\leqslant y$ in $P$ if and only if $S_x\subseteq S_y$. The cube height of a poset is the least non-negative integer $h$ such that $P$ has an inclusion representation for which every set has size at most $h$. In turn, the cube width of $P$ is the least non-negative integer $w$ for which there is an inclusion representation $\mathcal{S}$ of $P$ such that $|\bigcup\mathcal{S}|=w$ and every set in $\mathcal{S}$ has size at most the cube height of $P$. In this paper, we show that the cube width of a poset never exceeds the size of its ground set, and we characterize those posets for which this inequality is tight. Our research prompted us to investigate related extremal problems for posets and inclusion representations. Accordingly, the results for cube width are obtained as extensions of more comprehensive results that we believe to be of independent interest.

Figures (3)

• Figure 1: Inclusion representations of posets $P$ and $Q$ are shown. Sets are shown without braces and without commas. It is straightforward to verify that $\operatorname{ch}(P)=6$ and $\operatorname{cw}(P)=7<|P|=10$. Also, $\operatorname{ch}(Q)=5$ and $\operatorname{cw}(Q)=6<|Q|=8$.
• Figure 2: A poset in $\mathbb{MIIR}$ that is both a block and a component.
• Figure 3: A height $6$ poset in $\mathbb{MIIR}$ that is a block and a component.

Theorems & Definitions (44)

• Theorem 1
• Corollary 2
• Theorem 3
• Example 4
• Theorem 5
• Lemma 6
• proof
• proof : Proof of \ref{['thm:iir-bound']}
• Corollary 7
• proof