Haggkvist-Hell Graphs: A class of Kneser-colorable graphs
Abstract
For positive integers n and r we define the Haggkvist-Hell graph, Hn:r, to be the graph whose vertices are the ordered pairs (h,T) where T is an r-subset of [n], and h is an element of [n] not in T. Vertices (hx,Tx) and (hy,Ty) are adjacent iff hx ∈ Ty, hy ∈ Tx, and Tx and Ty are disjoint. These triangle-free arc transitive graphs are an extension of the idea of Kneser graphs, and there is a natural homomorphism from the Haggkvist-Hell graph, Hn:r, to the corresponding Kneser graph, Kn:r. Haggkvist and Hell introduced the r=3 case of these graphs, showing that a cubic graph admits a homomorphism to H22:3 if and only if it is triangle-free. Gallucio, Hell, and Nesetril also considered the r=3 case, proving that Hn:3 can have arbitrarily large chromatic number. In this paper we give the exact values for diameter, girth, and odd girth of all Haggkvist-Hell graphs, and we give bounds for independence, chromatic, and fractional chromatic number. Furthermore, we extend the result of Gallucio et al. to any fixed r 2, and we determine the full automorphism group of Hn:r, which is isomorphic to the symmetric group on n elements.