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Uncountable dichromatic number without short directed cycles

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arxiv 1905.00782 v3 pith:Z4NQRDAJ submitted 2019-05-02 math.CO math.LO

classification math.COmath.LO
keywords cyclesdichromaticdirectedkappanumbercontaindigraphomega
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abstract

A. Hajnal and P. Erd\H{o}s proved that a graph with uncountable chromatic number cannot avoid short cycles, it must contain for example $ C_4 $ (among other obligatory subgraphs). It was shown recently by D. T. Soukup that, in contrast of the undirected case, it is consistent that for any $ n<\omega $ there exists an uncountably dichromatic digraph without directed cycles shorter than $ n $. He asked if it is provable already in ZFC. We answer his question positively by constructing for every infinite cardinal $ \kappa $ and $ n<\omega $ a digraph of size $ 2^{\kappa} $ with dichromatic number at least $ \kappa^{+} $ which does not contain directed cycles of length less than $ n $ as a subdigraph.

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  1. On the growth rate of dichromatic numbers of finite subdigraphs

    math.CO 2019-08 conditional novelty 7.0 of 10

    For every growth function f, there are uncountably dichromatic digraphs of size continuum in which every (n+2)-dichromatic finite subdigraph has at least f(n) vertices, and it is consistent with arbitrarily large cont...

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