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What's the usual method for proving the criticality of a graph?

I've been trying out different methods and theorems but I can't find a decent method that's really convincing.

Thanks a lot in advance!

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Wondering what a critical graph is? Wikipedia tells us that a graph is critical if removing any vertex or edge results in the chromatic number decreasing (by one). – Yuval Filmus Feb 13 '11 at 22:56
A graph is critical if every one of its proper subgraphs (subgraph not equal to the original) has a chromatic color less than the original. – K-RAN Feb 13 '11 at 23:10
up vote 1 down vote accepted

If $G$ is $n$-critical (i.e. $\chi(G) = n$), then $\delta(G) \geq n-1$. So if $\delta(G) < n-1$ then $G$ is not $n$-critical.

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I'm going to assume that delta is the degree of any vertex? And is this an actual theorem? – K-RAN Feb 14 '11 at 1:27
If $G$ is $n$-critical then it is the complete graph. – Yuval Filmus Feb 14 '11 at 2:28
Yuval- the 5-cycle is 3-critical (deleting any vertex gives a bipartite graph), but it is not the complete graph. – Daniel Moskovich Nov 28 '11 at 14:16

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