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Assume I have a parametrized graph. The parameters are two integers $x$ and $y<x$. Let $S(x)=\{1, \ldots, x\}$. The vertices of the graph are all the subsets of $S(x)$ of size $y$. Two vertices share an edge if their intersection is empty. I need to find the chromatic number of this graph. Is this problem NP-hard ?

Thank you in advance for your help.

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    $\begingroup$ See en.wikipedia.org/wiki/Kneser_graph $\endgroup$ Dec 6, 2011 at 19:15
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    $\begingroup$ @AndreasBjörklund actualy I don't think you needed to shift this to a comment: your answer is exactly what the OP needs. $\endgroup$ Dec 6, 2011 at 19:27
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    $\begingroup$ Andreas pointed out the wikipedia page, and it says the chromatic number is x-2y+2. $\endgroup$ Dec 6, 2011 at 22:03
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    $\begingroup$ @AndreasBjörklund maybe you should undelete the answer now :) $\endgroup$ Dec 7, 2011 at 3:55
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    $\begingroup$ Sorry for misunderstanding your question, I should ask you before editing... Despite this, I am still curious about the chromatic index of Kneser graphs. $\endgroup$ Dec 7, 2011 at 4:37

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See http://en.wikipedia.org/wiki/Kneser_graph

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