Chromatic number of a particular graph Assume I have a parametrized graph. The parameters are two integers $x$ and $y&lt;x$.
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.
See http://en.wikipedia.org/wiki/Kneser_graph