Skip to main content

Section11.6The Probabilistic Method¶ permalink

At the outset of this chapter, we presented Erdős' original proof for the lower bound for the Ramsey number $R(n,n)$ using counting. Later, we recast the proof in a probabilistic setting. History has shown that this second perspective is the right one. To illustrate the power of this approach, we present a classic theorem, which is also due to Erdős, showing that there are graphs with large girth and large chromatic number.

The girth $g$ of a graph $G$ is the smallest integer for which $G$ contains a cycle on $g$ vertices. The girth of a forest is taken to be infinite, while the girth of a graph is three if and only if it has a triangle. You can check the families of triangle-free, large chromatic number, graphs constructed in Chapter 5 and see that each has girth four.

Subsection11.6.1Gaining Intuition with the Probabilistic Method

Experienced researchers are able to simplify the calculations in an argument of this type, as they know what can safely be discarded and what can not. Here's a quick tour of the essential steps. We want $E(X_1)$ to be small, so we set $n^se^{-ps^2}=1$ and get $s=\ln n/p\text{.}$ We want the number of small cycles to be about $n$ so we set $(gp)^g=n$ and get $p=n^{1/g-1}\text{.}$ Finally, we want $n=st$ which requires $n^{1/g}=t\text{.}$ The rest is just paying attention to details.