## A note on Hamiltonian circuits (and Uncle Paul)

Sometime back I read a short paper by Chvatal and Erdos that presents a couple of sufficiency conditions for hamiltonicity.

One of the results there is – If G is an s-connected graph with at least 3 vertices, and has a maximum independence number of s, then G has a hamiltonian circuit.

It is an interesting result connecting robustness and hamiltonicity. A graph is s-connected if there are s edge independent paths between any two nodes in the graph. In other words, it is the size of the minimum edge cut i.e. the minimum number of edges whose deletion increases the number of components in the graph.

The maximum independence number of a graph is the size of the biggest independent set. An independent set is a set of vertices of the graph such that no two vertices in the set are adjacent. In other words, there is no edge between any pair of vertices in the independent set. The bigger the independence number, the easier it is to fragment the network.

As you can see both these concepts can be used to measure the robustness of a graph. Also, it is not unnatural that connectivity and hamiltonicity are related. Intuitively, the more independent paths, the greater the chances of a graph being hamiltonian. And, a hamiltonian circuit is at least 2-connected: a circle is the simplest hamiltonian circuit and it is 2-connected.

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Well, but the point of this post is something else really. The paper I referred to in the beginning is just a 3-page note. And it has an interesting footnote on the 1st page. The footnote says: This note was written in Professor Richard K. Guy’s car on the way from Pullman to Spokane, Wash. The authors wish to express their gratitude to Mrs. Guy for smooth driving.