Embedding Graphs of Simple Treewidth into Sparse Products

TL;DR

This paper investigates how graphs with bounded simple treewidth can be embedded into the undirected graph of a directed product of two graphs, revealing bounds, constructions, and limitations related to treewidth and indegree constraints.

Contribution

It establishes that graphs of simple treewidth k can be embedded into directed products of graphs with bounded indegree and treewidth, and proves the optimality of these bounds.

Findings

Graphs of simple treewidth k embed into directed products with bounded indegree and treewidth.

Outerplanar graphs embed into products of trees with maximum indegree 1.

Certain graphs of treewidth k cannot be embedded into products with lower indegree constraints.

Abstract

We study embeddings of graphs with bounded treewidth or bounded simple treewidth into the undirected graph underlying the directed product of two directed graphs. If the factors have bounded maximum indegrees, then the product graph has bounded maximum indegree and therefore is sparse. We prove that every graph of simple treewidth $k$ is contained in (ignoring edge directions) the directed product of directed graphs $\vec H_1$ and $\vec H_2$, with $\Delta^-(\vec H_1), \Delta^-(\vec H_2) \leq k-1 $ and $\text{tw}(H_1), \text{tw}(H_2) \leq k-1$. Further, we show that this treewidth bound is best possible. Several corollaries follow from our results: every outerplanar graph is contained in the directed product of trees with maximum indegree $1$, and every planar graph with treewidth $3$ is contained in a directed product of graphs with treewidth $2$ and maximum indegree $2$. However, for graphs of treewidth $k$, we prove a negative result: for any integers $s, t, k \geq 1$, there is a graph $G$ with treewidth $k$ not contained in the directed product of $\vec H_1$ and $\vec H_2$ for any directed graphs $\vec H_1$ and $\vec H_2$ with $\Delta^-(\vec H_1) \leq s$, $\Delta^-(\vec H_2) \leq t$, and $\text{tw}(H_1), \text{tw}(H_2) \leq k-1$. This result stands in stark contrast to the strong product case, where Liu, Norin and Wood [arXiv:2410.20333] proved that the optimal lower bound on the factors is about half the treewidth.