Peter Rowlinson: A CHARACTERIZATION OF THE HOFFMAN-SINGLETON GRAPH

The Hoffman-Singleton graph [1], with spectrum $7^{(1)}$, $2^{28}$,
$(-3)^{21}$, is
characterized among
regular graphs by a star complement for the eigenvalue 2, that is, by an
induced subgraph
of order 22 without 2 as an eigenvalue.

REFERENCES

1. A. J. Hoffman and R. R. Singleton: On Moore graphs with diameters 2 and
3. IBM J. Res. Develop. 4 (1960), 497-504.