Decomposing Oriented Graphs into Six Locally Irregular Oriented Graphs

Abstract

An undirected graph G is locally irregular if every two of its adjacent vertices have distinct degrees. We say that G is decomposable into k locally irregular graphs if there exists a partition \(E_1 \cup E_2 \cup \cdots \cup E_k\) of the edge set E(G) such that each \(E_i\) induces a locally irregular graph. It was recently conjectured by Baudon et al. that every undirected graph admits a decomposition into at most three locally irregular graphs, except for a well-characterized set of indecomposable graphs. We herein consider an oriented version of this conjecture. Namely, can every oriented graph be decomposed into at most three locally irregular oriented graphs, i.e. whose adjacent vertices have distinct outdegrees? We start by supporting this conjecture by verifying it for several classes of oriented graphs. We then prove a weaker version of this conjecture. Namely, we prove that every oriented graph can be decomposed into at most six locally irregular oriented graphs. We finally prove that even if our conjecture were true, it would remain NP-complete to decide whether an oriented graph is decomposable into at most two locally irregular oriented graphs.

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Notes

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    Note that our investigations could have be done with respect to the indegrees instead.

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Acknowledgments

The authors thank the anonymous referees for their careful reading of a previous version of the current paper. J. Bensmail was supported by ERC Advanced Grant GRACOL, Project No. 320812.

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Correspondence to Julien Bensmail.

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Bensmail, J., Renault, G. Decomposing Oriented Graphs into Six Locally Irregular Oriented Graphs. Graphs and Combinatorics 32, 1707–1721 (2016). https://doi.org/10.1007/s00373-016-1705-z

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Keywords

  • Oriented graph
  • Locally irregular oriented graph
  • Decomposition into locally irregular graphs
  • Complexity