Horton graph

In the mathematical field of graph theory, the Horton graph or Horton 96-graph is a 3-regular graph with 96 vertices and 144 edges discovered by Joseph Horton.^[1] Published by Bondy and Murty in 1976, it provides a counterexample to the Tutte conjecture that every cubic 3-connected bipartite graph is Hamiltonian.^[2][3]

Horton graph
The Horton graph
Named after	Joseph Horton
Vertices	96
Edges	144
Radius	10
Diameter	10
Girth	6
Automorphisms	96 (Z/2Z×Z/2Z×S4)
Chromatic number	2
Chromatic index	3
Book thickness	3
Queue number	2
Properties	Cubic Bipartite
Table of graphs and parameters

After the Horton graph, a number of smaller counterexamples to the Tutte conjecture were found. Among them are a 92 vertex graph by Horton published in 1982, a 78 vertex graph by Owens published in 1983, and the two Ellingham-Horton graphs (54 and 78 vertices).^[4][5]

The first Ellingham-Horton graph was published by Ellingham in 1981 and was of order 78.^[6] At that time, it was the smallest known counterexample to the Tutte conjecture. The second one was published by Ellingham and Horton in 1983 and was of order 54.^[7] In 1989, Georges' graph, the smallest currently-known non-Hamiltonian 3-connected cubic bipartite graph was discovered, containing 50 vertices.^[8]

As a non-Hamiltonian cubic graph with many long cycles, the Horton graph provides good benchmark for programs that search for Hamiltonian cycles.^[9]

The Horton graph has chromatic number 2, chromatic index 3, radius 10, diameter 10, girth 6, book thickness 3^[10] and queue number 2.^[10] It is also a 3-edge-connected graph.

Algebraic properties

The automorphism group of the Horton graph is of order 96 and is isomorphic to Z/2Z×Z/2Z×S₄, the direct product of the Klein four-group and the symmetric group S₄.

The characteristic polynomial of the Horton graph is : ${\displaystyle (x-3)(x-1)^{14}x^{4}(x+1)^{14}(x+3)(x^{2}-5)^{3}(x^{2}-3)^{11}(x^{2}-x-3)(x^{2}+x-3)}$ ${\displaystyle (x^{10}-23x^{8}+188x^{6}-644x^{4}+803x^{2}-101)^{2}}$ ${\displaystyle (x^{10}-20x^{8}+143x^{6}-437x^{4}+500x^{2}-59)}$.

Gallery

• 
  The chromatic number of the Horton graph is 2.
• 
  The chromatic index of the Horton graph is 3.
• 
  The Ellingham-Horton 54-graph, a smaller counterexample to the Tutte conjecture.