Bounded-Degree Planar Graphs Do Not Have Bounded-Degree Product Structure

TL;DR

This paper proves that bounded-degree planar graphs cannot have their product structure bounded by the maximum degree, showing that certain structural bounds must grow super-polynomially with graph size.

Contribution

It demonstrates that no bounded-degree product structure theorem can be strengthened to keep the degree bound independent of the original graph's degree.

Findings

Existence of planar graphs with degree 5 requiring unbounded product parameters.

Super-polynomial lower bounds on product structure parameters for certain planar graphs.

Shows limitations of current product structure theorems for bounded-degree graphs.

Abstract

Product structure theorems are a collection of recent results that have been used to resolve a number of longstanding open problems on planar graphs and related graph classes. One particularly useful version states that every planar graph $G$ is contained in the strong product of a $3$-tree $H$, a path $P$, and a $3$-cycle $K_3$; written as $G\subseteq H\boxtimes P\boxtimes K_3$. A number of researchers have asked if this theorem can be strengthened so that the maximum degree in $H$ can be bounded by a function of the maximum degree in $G$. We show that no such strengthening is possible. Specifically, we describe an infinite family $\mathcal{G}$ of planar graphs of maximum degree $5$ such that, if an $n$-vertex member $G$ of $\mathcal{G}$ is isomorphic to a subgraph of $H\boxtimes P\boxtimes K_c$ where $P$ is a path and $H$ is a graph of maximum degree $\Delta$ and treewidth $t$, then $t\Delta c \ge 2^{\Omega(\sqrt{\log\log n})}$.