Local chromatic number and distinguishing the strength of topological obstructions
Gábor Simonyi, Gábor Tardos, Siniša T. Vrećica · Transactions of the American Mathematical Society · 2008
The local chromatic number of a graph G G is the number of colors appearing in the most colorful closed neighborhood of a vertex minimized over all proper colorings of G G . We show that two specific topological obstructions that have the same implications for the chromatic number have different implications for the local chromatic number. These two obstructions can be formulated in terms of the homomorphism complex Hom ( K 2 , G ) \textrm {Hom}(K_2,G) and its suspension, respectively. These investigations follow the line of research initiated by Matoušek and Ziegler who recognized a hierarchy of the different topological expressions that can serve as lower bounds for the chromatic number of a graph. Our results imply that the local chromatic number of 4 4 -chromatic Kneser, Schrijver, Borsuk, and generalized Mycielski graphs is 4 4 , and more generally, that 2 r 2r -chromatic versions of these graphs have local chromatic number at least r + 2 r+2 . This lower bound is tight in several cases by results of the first two authors.