Abstract
We study 2k-factors in \((2r+1)\)-regular graphs. Hanson, Loten, and Toft proved that every \((2r+1)\)-regular graph with at most 2r cut-edges has a 2-factor. We generalize their result by proving for \(k\le (2r+1)/3\) that every \((2r+1)\)-regular graph with at most \(2r-3(k-1)\) cut-edges has a 2k-factor. Both the restriction on k and the restriction on the number of cut-edges are sharp. We characterize the graphs that have exactly \(2r-3(k-1)+1\) cut-edges but no 2k-factor. For \(k>(2r+1)/3\), there are graphs without cut-edges that have no 2k-factor, as studied by Bollobás, Saito, and Wormald.


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Funding
Alexandr V. Kostochka: Research supported by National Science Foundation grant DMS1600592 and grants 18-01-00353A and 19-01-00682 of the Russian Foundation for Basic Research. Douglas B. West: Research supported by National Natural Science Foundation of China grants NSFC 11871439 and 11971429. Dara Zirlin: Research supported by Arnold O. Beckman Campus Research Board Award RB20003 of the University of Illinois at Urbana-Champaign.
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Kostochka, A.V., Raspaud, A., Toft, B. et al. Cut-Edges and Regular Factors in Regular Graphs of Odd Degree. Graphs and Combinatorics 37, 199–207 (2021). https://doi.org/10.1007/s00373-020-02242-0
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DOI: https://doi.org/10.1007/s00373-020-02242-0