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Enumerating the edge-colourings and total colourings of a regular graph

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Abstract

In this paper, we are interested in computing the number of edge colourings and total colourings of a connected graph. We prove that the maximum number of k-edge-colourings of a connected k-regular graph on n vertices is k⋅((k−1)!)n/2. Our proof is constructive and leads to a branching algorithm enumerating all the k-edge-colourings of a connected k-regular graph in time O (((k−1)!)n/2) and polynomial space. In particular, we obtain a algorithm to enumerate all the 3-edge-colourings of a connected cubic graph in time O (2n/2)=O (1.4143n) and polynomial space. This improves the running time of O (1.5423n) of the algorithm due to Golovach et al. (Proceedings of WG 2010, pp. 39–50, 2010). We also show that the number of 4-total-colourings of a connected cubic graph is at most 3⋅23n/2. Again, our proof yields a branching algorithm to enumerate all the 4-total-colourings of a connected cubic graph.

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Acknowledgement

The authors would like to thank their children for suggesting them some graph names.

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Correspondence to F. Havet.

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This work is supported by the French Agence Nationale de la Recherche under reference AGAPE ANR-09-BLAN-0159.

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Bessy, S., Havet, F. Enumerating the edge-colourings and total colourings of a regular graph. J Comb Optim 25, 523–535 (2013). https://doi.org/10.1007/s10878-011-9448-5

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