Trade-off between spread and width for tree decompositions

TL;DR

This paper investigates the relationship between spread and width in tree decompositions, establishing bounds on how spread can be controlled relative to treewidth, and demonstrating near-optimal spread with bounded width.

Contribution

It proves necessary and sufficient conditions for controlling spread relative to treewidth in tree decompositions, and shows near-optimal spread can be achieved with bounded width.

Findings

C ≥ 2 is necessary for controlling spread.

C > 3 is sufficient for controlling spread.

Near-optimal average spread can be achieved with width proportional to treewidth.

Abstract

We study the trade-off between (average) spread and width in tree decompositions, answering several questions from Wood [arXiv:2509.01140]. The spread of a vertex $v$ in a tree decomposition is the number of bags that contain $v$. Wood asked for which $c>0$, there exists $c'$ such that each graph $G$ has a tree decomposition of width $c\cdot tw(G)$ in which each vertex $v$ has spread at most $c'(d(v)+1)$. We show that $c\geq 2$ is necessary and that $c>3$ is sufficient. Moreover, we answer a second question fully by showing that near-optimal average spread can be achieved simultaneously with width $O(tw(G))$.