Subchromatic numbers of powers of graphs with excluded minors
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A k-subcolouring of a graph G is a function f:V(G) →{0,…,k-1} such that the set of vertices coloured i induce a disjoint union of cliques. The subchromatic number, χ_sub(G), is the minimum k such that G admits a k-subcolouring. Nešetřil, Ossona de Mendez, Pilipczuk, and Zhu (2020), recently raised the problem of finding tight upper bounds for χ_sub(G^2) when G is planar. We show that χ_sub(G^2)≤ 43 when G is planar, improving their bound of 135. We give even better bounds when the planar graph G has larger girth. Moreover, we show that χ_sub(G^3)≤ 95, improving the previous bound of 364. For these we adapt some recent techniques of Almulhim and Kierstead (2022), while also extending the decompositions of triangulated planar graphs of Van den Heuvel, Ossona de Mendez, Quiroz, Rabinovich and Siebertz (2017), to planar graphs of arbitrary girth. Note that these decompositions are the precursors of the graph product structure theorem of planar graphs. We give improved bounds for χ_sub(G^p) for all p, whenever G has bounded treewidth, bounded simple treewidth, bounded genus, or excludes a clique or biclique as a minor. For this we introduce a family of parameters which form a gradation between the strong and the weak colouring numbers. We give upper bounds for these parameters for graphs coming from such classes. Finally, we give a 2-approximation algorithm for the subchromatic number of graphs coming from any fixed class with bounded layered cliquewidth. In particular, this implies a 2-approximation algorithm for the subchromatic number of powers G^p of graphs coming from any fixed class with bounded layered treewidth (such as the class of planar graphs). This algorithm works even if the power p and the graph G is unknown.
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