# Robertson graph

Robertson graph
The Robertson graph is Hamiltonian.
Named after Neil Robertson
Vertices 19
Edges 38
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 Robertson in 1964.[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).\$

## References

1. ^
2. ^
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.