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The Grötzsch graph is triangle-free and has chromatic number 4. At 11 vertices it is the (unique) smallest graph with these properties.

What is the smallest number of vertices needed for a triangle-free graph with chromatic number 5? The Mycielskian of the Grötzsch graph has 23 vertices, so it's not larger than that.

There are far too many triangle-free connected graphs for enumeration to be a viable strategy. This question may well be unsolved; if so, I'd love a citation saying as much (if such exists).

Reed's conjecture implies that any such graph has max degree $\Delta\ge6.$

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22 vertices, there are 80 of them.

Jensen and Royle, Small graphs with chromatic number 5 : a computer search Journal of Graph Theory, 1995.

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    $\begingroup$ Volume 19, Issue 1, January 1995, pp 107-116. $\endgroup$ – Gerry Myerson Feb 5 '18 at 22:34
  • $\begingroup$ Note: The paper finds 80 graphs but doesn’t claim completeness. $\endgroup$ – Charles Feb 6 '18 at 7:27
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    $\begingroup$ I’ve redone it recently and confirmed that 80 is correct (should have said that in original answer, but can’t type well on small screen). $\endgroup$ – Gordon Royle Feb 6 '18 at 7:42
  • $\begingroup$ @GordonRoyle Do you mind if I quote you on that? $\endgroup$ – Charles Feb 13 '18 at 21:04
  • $\begingroup$ @Charles, feel free to quote me. $\endgroup$ – Gordon Royle Feb 13 '18 at 23:17

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