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Unformatted text preview: MATH 682 Notes Combinatorics and Graph Theory II 1 Advanced Chromatic Properties 1.1 ColorCriticality When we were proving particular results in chromatic number, such as the fivecolor theorem, we frequently assumed we were looking at a minimal example of some species of graph. Very often, the minimal examples of a problem have exploitable structures we can make better use of. Such graphs are called critical : Definition 1. A graph G without isolated vertices is colorcritical if ( G e ) < ( G ) for any e E ( G ). If ( G ) = k , this property may be further described as kcriticality . We know a few colorcritical graphs off the top of our heads: K n will be ncritical, since removal of any edge allows coloring with n 1 colors, and C 2 n +1 will be 3critical, since removing any edge admits a 2coloring. In fact, every graph has a kcritical core, which in many cases is not a cycle or a clique. Proposition 1. If ( G ) = k , then G has a kcritical subgraph. Proof. Let us prove this by induction on k G k ; for the base case, note that k G k = 1 corresponds uniquely to a 2critical graph. For larger G , one of two things is true: either G is kcritical, in which case it is its own k critical subgraph, or there is an edge e such that ( G e ) ( G ). Since any coloring of G e is a proper coloring of G , we know that this nonstrict inequality is in fact an equality; that is, ( G e ) = ( G ) = k . Then, by the inductive hypothesis, since k G e k < k G k and ( G e ) = k , G e has a kcritical subgraph, which is in turn a kcritical subgraph of G . Since we now know kcritical graphs are all over the place, we might begin to wonder about their structure, since they are exemplars of the necessary substructures for a graph to require k colors. Proposition 2. If G is kcritical, then ( G ) k 1 . Proof. Suppose ( G ) = k and G has a vertex v with degree less than k 1. Let e = { u,v } for some neighbor u of v . If ( G e ) = k 1, then it must follow that in this coloring u and v are the same color (or this would be a ( k 1)coloring of G as well; however, since v has no more than k 3 neighbors in G e , there must be two colors k 1 not represented in its neighborhood, so any proper coloring of G e except for v allows at least two choices of color for v , so it can always be selected to be a different color than u , inducing a ( k 1)coloring of G , which contradicts the given chromatic number of G . Thus, ( G e ) must be k , so G is not kcritical. Not only is the minimum degree at each vertex necessarily close to the chromatic number of a critical graph, but in fact, the more global concept of edgeconnectivity must also be dictated by the chromatic number. We might start with a quite simple observation....
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 Spring '09
 WILDSTROM
 Combinatorics, Graph Theory

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