Characterization of k-chromatic graphs

Characterization of k-chromatic graphs [closed]

I would like to make a collection of the known different characterization of k-chromatic graphs. Please contribute by posting your favorites.

flag	asked May 28 2012 at 1:06 hbm 188●1●6

	I am not sure what the question is asking for, and what of it cannot be got by googling "graph coloring". Voting to close. – Igor Rivin May 28 2012 at 1:10
	This question is too broad. Please read the FAQ mathoverflow.net/faq – Gjergji Zaimi May 28 2012 at 1:13