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Published online by Cambridge University Press: 28 April 2025
We study several basic problems about colouring the  $p$-random subgraph
$p$-random subgraph  $G_p$ of an arbitrary graph
$G_p$ of an arbitrary graph  $G$, focusing primarily on the chromatic number and colouring number of
$G$, focusing primarily on the chromatic number and colouring number of  $G_p$. In particular, we show that there exist infinitely many
$G_p$. In particular, we show that there exist infinitely many  $k$-regular graphs
$k$-regular graphs  $G$ for which the colouring number (i.e., degeneracy) of
$G$ for which the colouring number (i.e., degeneracy) of  $G_{1/2}$ is at most
$G_{1/2}$ is at most  $k/3 + o(k)$ with high probability, thus disproving the natural prediction that such random graphs must have colouring number at least
$k/3 + o(k)$ with high probability, thus disproving the natural prediction that such random graphs must have colouring number at least  $k/2 - o(k)$.
$k/2 - o(k)$.
 $\lambda _1,$
 isoperimetric inequalities for graphs, and superconcentrators. J. Combin. Theory Ser. B. 38 73–88.CrossRefGoogle Scholar
$\lambda _1,$
 isoperimetric inequalities for graphs, and superconcentrators. J. Combin. Theory Ser. B. 38 73–88.CrossRefGoogle Scholar $G_{n,p}$
. Ann. Appl. Probab. 22 1989–2047.CrossRefGoogle Scholar
$G_{n,p}$
. Ann. Appl. Probab. 22 1989–2047.CrossRefGoogle Scholar