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Colouring planar graphs with bounded monochromatic components. (English) Zbl 1440.05086
Summary: O. V. Borodin and A. O. Ivanova [J. Graph Theory 67, No. 2, 83–90 (2011; Zbl 1218.05038)] proved that every planar graph of girth at least 7 is 2-choosable with the property that each monochromatic component is a path with at most 3 vertices. M. Axenovich et al. [J. Graph Theory 85, No. 3, 601–618 (2017; Zbl 1367.05044)] proved that every planar graph of girth 6 is 2-choosable so that each monochromatic component is a path with at most 15 vertices. We improve both these results by showing that planar graphs of girth at least 6 are 2-choosable so that each monochromatic component is a path with at most 3 vertices. Our second result states that every planar graph of girth 5 is 2-choosable so that each monochromatic component is a tree with at most 515 vertices. Finally, we prove that every graph with fractional arboricity at most $$\frac{2d+2}{d+2}$$ is 2-choosabale with the property that each monochromatic component is a tree with maximum degree at most $$d$$. This implies that planar graphs of girth 5, 6, and 8 are 2-choosable so that each monochromatic component is a tree with maximum degree at most 4, 2, and 1, respectively. All our results are obtained by applying the nine dragon tree theorem, which was recently proved by H. Jiang and D. Yang [Combinatorica 37, No. 6, 1125–1137 (2017; Zbl 1399.05034)], and the strong nine dragon tree conjecture partially confirmed by S.-J. Kim et al. [J. Graph Theory 74, No. 3–4, 369–391 (2013; Zbl 1276.05092)] and B. Moore [“An approximate version of the strong nine dragon tree conjecture”, Preprint, arXiv:1909.07946].
##### MSC:
 05C15 Coloring of graphs and hypergraphs 05C10 Planar graphs; geometric and topological aspects of graph theory 05C70 Edge subsets with special properties (factorization, matching, partitioning, covering and packing, etc.)
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