Abstract
An r-augmented tree is a rooted tree plus r edges added from each leaf to ancestors. For d, g, r ∈ N, we construct a bipartite r-augmented complete d-ary tree having girth at least g. The height of such trees must grow extremely rapidly in terms of the girth.
Using the resulting graphs, we construct sparse non-k-choosable bipartite graphs, showing that maximum average degree at most 2(k - 1) is a sharp sufficient condition for k-choosability in bipartite graphs, even when requiring large girth. We also give a new simple construction of non-k-colorable graphs and hypergraphs with any girth.
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Research supported in part by a USA-Israeli BSF grant, by an ISF grant, by the Israeli I-Core program and by the Oswald Veblen Fund.
Research supported in part by NSF grant DMS-1266016.
Research supported by Recruitment Program of Foreign Experts, 1000 Talent Plan, State Administration of Foreign Experts Affairs, China.
Research supported by CNSF 11571319.
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Alon, N., Kostochka, A., Reiniger, B. et al. Coloring, sparseness and girth. Isr. J. Math. 214, 315–331 (2016). https://doi.org/10.1007/s11856-016-1361-2
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DOI: https://doi.org/10.1007/s11856-016-1361-2