Decomposing Degenerate Graphs into Locally Irregular Subgraphs


A (undirected) graph is locally irregular if no two of its adjacent vertices have the same degree. A decomposition of a graph G into k locally irregular subgraphs is a partition \(E_1,\dots ,E_k\) of E(G) into k parts each of which induces a locally irregular subgraph. Not all graphs decompose into locally irregular subgraphs; however, it was conjectured that, whenever a graph does, it should admit such a decomposition into at most three locally irregular subgraphs. This conjecture was verified for a few graph classes in recent years. This work is dedicated to the decomposability of degenerate graphs with low degeneracy. Our main result is that decomposable k-degenerate graphs decompose into at most \(3k+1\) locally irregular subgraphs, which improves on previous results whenever \(k \le 9\). We improve this result further for some specific classes of degenerate graphs, such as bipartite cacti, k-trees, and planar graphs. Although our results provide only little progress towards the leading conjecture above, the main contribution of this work is rather the decomposition schemes and methods we introduce to prove these results.

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Correspondence to Julien Bensmail.

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This work has been supported by the French government, through the \(\hbox {UCA}^\textsc {jedi}\) Investments in the Future project managed by the National Research Agency (ANR) with the reference number ANR-15-IDEX-01, by the ANR project Digraphs with the reference number ANR-19-CE48-0013, and by the STIC-AmSud Program with the project GALOP.

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Bensmail, J., Dross, F. & Nisse, N. Decomposing Degenerate Graphs into Locally Irregular Subgraphs. Graphs and Combinatorics 36, 1869–1889 (2020).

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  • Locally irregular decompositions
  • Degenerate graphs
  • Cacti
  • k-trees
  • Planar graphs