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## Connected greedy colourings of perfect graphs and other classes: the good, the bad and the ugly

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Laurent Beaudou
Florent Foucaud

#### Abstract

The Grundy number of a graph is the maximum number of colours used by the First-Fit'' greedy colouring algorithm over all vertex orderings. Given a vertex ordering $\sigma= v_1,\dots,v_n$, the First-Fit'' greedy colouring algorithm colours the vertices in the order of $\sigma$ by assigning to each vertex the smallest colour unused in its neighbourhood. By restricting this procedure to vertex orderings that are connected, we obtain {\em connected greedy colourings}. For some graphs, all connected greedy colourings use exactly $\chi(G)$ colours; they are called {\em good graphs}. On the opposite, some graphs do not admit any connected greedy colouring using only $\chi(G)$ colours; they are called {\em ugly graphs}. We show that no perfect graph is ugly. We also give simple proofs of this fact for subclasses of perfect graphs (block graphs, comparability graphs), and show that no $K_4$-minor free graph is ugly.

### Dates and versions

hal-03406709 , version 1 (28-10-2021)

### Identifiers

• HAL Id : hal-03406709 , version 1
• ARXIV :

### Cite

Laurent Beaudou, Caroline Brosse, Oscar Defrain, Florent Foucaud, Aurélie Lagoutte, et al.. Connected greedy colourings of perfect graphs and other classes: the good, the bad and the ugly. 2021. ⟨hal-03406709⟩

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