Using a Zq-generalization of a theorem of Ky Fan, we extend to Kneser hypergraphs a theorem of Simonyi and Tardos that ensures the existence of multicolored complete bipartite graphs in any proper coloring of a Kneser graph. It allows to derive a lower bound for the local chromatic number of Kneser hypergraphs (using a natural definition of what can be the local chromatic number of a uniform hy...

The Kneser graph Kn:k for positive integers n ≥ k has as its vertex set the k-element subsets of some n-set, with disjoint sets being adjacent. Every finite simple graph can be found as an induced subgraph of some Kneser graph; this can be viewed as a way of representing graphs by labelling their vertices with sets. We explore questions of finding the smallest representation (both in terms of n...

This paper proves that for any positive integer n, if m is large enough, then the reduced Kneser graph KG2(m, n) has its circular chromatic number equal its chromatic number. This answers a question of Lih and Liu [J. Graph Theory, 2002]. For Kneser graphs, we prove that if m ≥ 2n2(n − 1), then KG(m, n) has its circular chromatic number equal its chromatic number. This provides strong support f...

The vertex set of a Kneser graph KG(m,n) consists of all n-subsets of the set [m] = {0, 1, . . . ,m − 1}. Two vertices are defined to be adjacent if they are disjoint as subsets. A subset of [m] is called 2stable if 2 ≤ |a − b| ≤ m − 2 for any distinct elements a and b in that subset. The reduced Kneser graph KG2(m,n) is the subgraph of KG(m,n) induced by vertices that are 2-stable subsets. We ...

A graph G is said to be hom-idempotent if there is a homomorphism from G2 to G, and weakly hom-idempotent if for some n ≥ 1 there is a homomorphism from Gn+1 to Gn. Larose et al. (1998) proved that Kneser graphs KG(n, k) are not weakly hom-idempotent for n ≥ 2k + 1, k ≥ 2. For s ≥ 2, we characterize all the shifts (i.e., automorphisms of the graph that map every vertex to one of its neighbors) ...

The Kneser conjecture (1955) was proved by Lovász (1978) using the Borsuk-Ulam theorem; all subsequent proofs, extensions and generalizations also relied on Algebraic Topology results, namely the Borsuk-Ulam theorem and its extensions. Only in 2000, Matoušek provided the first combinatorial proof of the Kneser conjecture. Here we provide a hypergraph coloring theorem, with a combinatorial proof...

The Shannon capacity of a graph G is c(G) = supd>1(α(G d)) 1 d , where α(G) is the independence number of G. The Shannon capacity of the Kneser graph KGn,r was determined by Lovász in 1979, but little is known about the Shannon capacity of the complement of that graph when r does not divide n. The complement of the Kneser graph, KGn,2, is also called the triangular graph Tn. The graph Tn has th...

A set of vertices S is a determining set for a graph G if every automorphism of G is uniquely determined by its action on S. The determining number of a graph is the size of a smallest determining set. This paper describes ways of finding and verifying determining sets, gives natural lower bounds on the determining number, and shows how to use orbits to investigate determining sets. Further, de...

We determine the maximal cocliques of size ≥ 4q2 + 5q + 5 in the Kneser graph on point-plane flags in PG(4, q). The maximal size of a coclique in this graph is (q2 + q + 1)(q3 + q2 + q + 1).

In this paper, we investigate circular chromatic number of Mycielski construction of graphs. It was shown in [20] that t Mycielskian of the Kneser graph KG(m,n) has the same circular chromatic number and chromatic number provided that m + t is an even integer. We prove that if m is large enough, then χ(M (KG(m,n))) = χc(M (KG(m,n))) where M t is t Mycielskian. Also, we consider the generalized ...