Coarse tree-width

Abstract: We prove two theorems about tree-decompositions in the setting of coarse graph theory. First, we show that a graph $G$ admits a tree-decomposition in which each bag is contained in the union of a bounded number of balls of bounded radius, if and only if $G$ admits a quasi-isometry to a graph with bounded tree-width. (The if'' half is easy, but theonly if'' half is challenging.) This generalizes a recent result of Berger and Seymour, concerning tree-decompositions when each bag has bounded radius. Second, we show that if $G$ admits a quasi-isometry $\phi$ to a graph $H$ of bounded path-width, then $G$ admits a quasi-isometry (with error only an additive constant) to a graph of bounded path-width. Indeed, we will show a much stronger statement: that we can assign a non-negative integer length to each edge of $H$, such that the same function $\phi$ is a quasi-isometry (with error only an additive constant) to this weighted version of $H$.