Geometric realizations of dichotomous ordinal graphs
Patrizio Angelini, Sabine Cornelsen, Carolina Haase, Michael Hoffmann, Eleni Katsanou, Fabrizio Montecchiani, Raphael Steiner, Antonios Symvonis
TL;DR
This work develops the theory of pandichotomic dimensions for graphs by linking degeneracy to pandichotomous realizability in both Euclidean and spherical spaces. It proves that every $d$-degenerate graph is pandichotomous in $\mathbb{R}^d$ and in $\mathbb{S}^{d-1}$, with tightness statements, and establishes a linear edge bound $m \le \mu d n$ for pandichotomous graphs in $\mathbb{R}^d$. It fully characterizes pandichotomous complete bipartite graphs in $\mathbb{R}^2$ and provides robust realizability guarantees for bipartite graphs under structural constraints on the short or long subgraphs. The results reveal a tight, dimensionally linear relationship between pandichotomic dimension and degeneracy, while also outlining several open questions on planar and higher-dimensional realizability.
Abstract
A dichotomous ordinal graph consists of an undirected graph with a partition of the edges into short and long edges. A geometric realization of a dichotomous ordinal graph $G$ in a metric space $X$ is a drawing of $G$ in $X$ in which every long edge is strictly longer than every short edge. We call a graph $G$ pandichotomous in $X$ if $G$ admits a geometric realization in $X$ for every partition of its edge set into short and long edges. We exhibit a very close relationship between the degeneracy of a graph $G$ and its pandichotomic Euclidean or spherical dimension, that is, the smallest dimension $k$ such that $G$ is pandichotomous in $\mathbb{R}^k$ or the sphere $\mathbb{S}^k$, respectively. First, every $d$-degenerate graph is pandichotomous in $\mathbb{R}^{d}$ and $\mathbb{S}^{d-1}$ and these bounds are tight for the sphere and for $\mathbb{R}^2$ and almost tight for $\mathbb{R}^d$, for $d\ge 3$. Second, every $n$-vertex graph that is pandichotomous in $\mathbb{R}^k$ has at most $μkn$ edges, for some absolute constant $μ<7.23$. This shows that the pandichotomic Euclidean dimension of any graph is linearly tied to its degeneracy and in the special cases $k\in \{1,2\}$ resolves open problems posed by Alam, Kobourov, Pupyrev, and Toeniskoetter. Further, we characterize which complete bipartite graphs are pandichotomous in $\mathbb{R}^2$: These are exactly the $K_{m,n}$ with $m\le 3$ or $m=4$ and $n\le 6$. For general bipartite graphs, we can guarantee realizations in $\mathbb{R}^2$ if the short or the long subgraph is constrained: namely if the short subgraph is outerplanar or a subgraph of a rectangular grid, or if the long subgraph forms a caterpillar.