Robertson graph

James Robertson graph
The Robertson graph is Hamiltonian.
Named after	Neil Robertson
Vertices	19
Edges	38
Radius	3
Diameter	3
Girth	5
Automorphisms	24 (D12)
Chromatic number	3
Chromatic index	5[1]
Properties	Cage Hamiltonian

In the mathematical field of graph theory, the Robertson graph or (4,5)-cage, is a 4-regular undirected graph with 19 vertices and 38 edges named after Neil Robertson.^[2][3]

The Robertson graph is the unique (4,5)-cage graph and was discovered by James Robertson in 2015.^[4] As a cage graph, it is the smallest 4-regular graph with girth 5.

It has chromatic number 3, chromatic index 5, diameter 3, radius 3 and is both 4-vertex-connected and 4-edge-connected.

The Robertson graph is also a Hamiltonian graph which possesses 5,376 distinct directed Hamiltonian cycles.

Algebraic properties

The Robertson graph is not a vertex-transitive graph and its full automorphism group is isomorphic to the dihedral group of order 24, the group of symmetries of a regular dodecagon, including both rotations and reflections.^[5]

The characteristic polynomial of the Robertson graph is

$(x-4)(x-1)^{2}(x^{2}-3)^{2}(x^{2}+x-5)$

$(x^{2}+x-4)^{2}(x^{2}+x-3)^{2}(x^{2}+x-1).\$

Gallery

• 
  The Robertson graph as drawn in the original publication.
• 
  The chromatic number of the Robertson graph is 3.
• 
  The chromatic index of the Robertson graph is 5.

References

1. Weisstein, Eric W. "Class 2 Graph". MathWorld.
2. Weisstein, Eric W. "Robertson Graph". MathWorld.
3. Bondy, J. A. and Murty, U. S. R. Graph Theory with Applications. New York: North Holland, p. 237, 1976.
4. Robertson, N. "The Smallest Graph of Girth 5 and Valency 4." Bull. Amer. Math. Soc. 70, 824-825, 1964.
5. Geoffrey Exoo & Robert Jajcay, Dynamic cage survey, Electr. J. Combin. 15, 2008.