# Biggs–Smith graph

In the mathematical field of graph theory, the BiggsSmith graph is a 3-regular graph with 102 vertices and 153 edges.[1]

BiggsSmith graph
The BiggsSmith graph
Vertices102
Edges153
Diameter7
Girth9
Automorphisms2448 (PSL(2,17))
Chromatic number3
Chromatic index3
PropertiesSymmetric
Distance-regular
Cubic
Hamiltonian
Table of graphs and parameters

It has chromatic number 3, chromatic index 3, radius 7, diameter 7 and girth 9. It is also a 3-vertex-connected graph and a 3-edge-connected graph.

All the cubic distance-regular graphs are known.[2] The BiggsSmith graph is one of the 13 such graphs.

## Algebraic properties

The automorphism group of the BiggsSmith graph is a group of order 2448[3] isomorphic to the projective special linear group PSL(2,17). It acts transitively on the vertices, on the edges and on the arcs of the graph. Therefore, the BiggsSmith graph is a symmetric graph. It has automorphisms that take any vertex to any other vertex and any edge to any other edge. According to the Foster census, the Biggs–Smith graph, referenced as F102A, is the only cubic symmetric graph on 102 vertices.[4]

The BiggsSmith graph is also uniquely determined by its graph spectrum, the set of graph eigenvalues of its adjacency matrix.[5]

The characteristic polynomial of the BiggsSmith graph is : ${\displaystyle (x-3)(x-2)^{18}x^{17}(x^{2}-x-4)^{9}(x^{3}+3x^{2}-3)^{16}}$.

## References

1. Weisstein, Eric W. "BiggsSmith Graph". MathWorld.
2. Brouwer, A. E.; Cohen, A. M.; and Neumaier, A. Distance-Regular Graphs. New York: Springer-Verlag, 1989.
3. Royle, G. F102A data
4. Conder, M. and Dobcsányi, P. "Trivalent Symmetric Graphs Up to 768 Vertices." J. Combin. Math. Combin. Comput. 40, 4163, 2002.
5. E. R. van Dam and W. H. Haemers, Spectral Characterizations of Some Distance-Regular Graphs. J. Algebraic Combin. 15, pages 189202, 2003
• On trivalent graphs, NL Biggs, DH Smith - Bulletin of the London Mathematical Society, 3 (1971) 155-158.