Is there a characterization of graphs $G$ such that $\exists$ $\phi : G \rightarrow KG(n,k)$, where $KG(n,k)$ is the Kneser graph ($k \leq \lceil \frac{n}{2}\rceil $).


Any references on the subject will be appreciated.



Homomorphism into Kneser  graphs  $KG(n, k)$
Homomorphisms into Kneser graphs are another way of describing fractional colourings; an introduction to how this all works is the topic of one of the chapters in my favourite book on Algebraic Graph Theory. There are other much more detailed references on fractional colourings, but not necessarily from the homomorphism viewpoint.


But just as there is no useful "characterization" of, say, graphs with 3-colourings (unless P=NP) there is no characterization of graphs with a given fractional chromatic number (except for a few trivial cases).


