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    Chromatic-choosability of hypergraphs with high chromatic number

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    It was conjectured by Ohba and confirmed recently by Noel et al. that, for any graph GG, if ∣V(G)βˆ£β‰€2Ο‡(G)+1|V(G)|\le 2\chi(G)+1 then Ο‡l(G)=Ο‡(G)\chi_l(G)=\chi(G). This indicates that the graphs with high chromatic number are chromatic-choosable. We show that this is also the case for uniform hypergraphs and further propose a generalized version of Ohba's conjecture: for any rr-uniform hypergraph HH with rβ‰₯2r\geq 2, if ∣V(H)βˆ£β‰€rΟ‡(H)+rβˆ’1|V(H)|\le r\chi(H)+r-1 then Ο‡l(H)=Ο‡(H)\chi_l(H)=\chi(H). We show that the condition of the proposed conjecture is sharp by giving two classes of rr-uniform hypergraphs HH with ∣V(H)∣=rΟ‡(H)+r|V(H)|= r\chi(H)+r and Ο‡l(H)>Ο‡(H)\chi_l(H)>\chi(H). To support the conjecture, we give two classes of rr-uniform hypergraphs HH with ∣V(H)∣=rΟ‡(H)+rβˆ’1|V(H)|= r\chi(H)+r-1 and prove that Ο‡l(H)=Ο‡(H)\chi_l(H)=\chi(H).Comment: 18 page
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