chromatic number and girth
It seems intuitively plausible that a high chromatic number occurs because of short, “local” cycles in the graph; it is hard to envisage how a graph with no short cycles can still have high chromatic number.
Instead of envisaging, Erdős’ proof shows that, in some appropriately chosen probability space on graphs with vertices, the probability of choosing a graph which does not have and tends to zero as grows. In particular, the desired graphs exist.
This seminal paper is probably the most famous application of the probabilistic method, and is regarded by some as the foundation of the method.22However, as always, with the benefit of hindsight we can see that the probabilistic method had been used before, e.g. in various applications of Sard’s theorem. This does nothing to diminish from the importance of the clear statement of the tool. Today the probabilistic method is a standard tool for combinatorics. More constructive methods are often preferred, but are almost always much harder.
|Title||chromatic number and girth|
|Date of creation||2013-03-22 12:46:03|
|Last modified on||2013-03-22 12:46:03|
|Last modified by||mathcam (2727)|