Treewidth and the Product of Tree-Independence and Tree-Chromatic Numbers

Source abstract

For a graph $G$, let $\mathrm{tw}(G)$, $\mathrm{tree}\text{-}α(G)$, and $\mathrm{tree}\text{-}χ(G)$ denote its treewidth, tree-independence number, and tree-chromatic number, respectively. We prove that every nonempty graph $G$ satisfies $\mathrm{tw}(G)+1\leq\mathrm{tree}\text{-}α(G)\mathrm{tree}\text{-}χ(G)(1+\lfloor\log_2(\mathrm{tree}\text{-}α(G))\rfloor)$. This improves the quadratic bound of Koutsoutis, Krause, Liu, Redzic, and Ueckerdt. For every pair of integers $n,k\geq2$, we construct a graph $G$ with $\mathrm{tree}\text{-}α(G)=n$, $\mathrm{tree}\text{-}χ(G)=k$, and $\mathrm{tw}(G)=Ω(nk\log_2 n)$. Thus the upper bound is best possible up to a constant factor.