On tree decompositions whose trees are subgraphs

Fix $k\in \mathbb{N}$ and let $G$ be a connected graph with treewidth at most $k$. We say that $xy\mathrm{\notin }E(G)$ is a {\em $k$-ghost-edge} of $G$ if for every tree decomposition $(T,\text{cB})$ of $G$ with width at most $k$, both $x$ and $y$ are contained in a bag of $(T,\text{cB})$. Moreover, if $G$ does not contain any $k$-ghost-edges, then $G$ is {\em $k$-ghost-free}. Hickingbotham proposed a conjecture that every connected $k$-ghost-free graph $G$ has a tree decomposition $(T,\text{cB})$ with width at most $k$ such that $T$ is a subgraph of $G$. In this paper, we prove that Hickingbotham’s conjecture is false for all $k\ge 3$.