We give a short and alternative proof of a theorem of F. Jaeger that except for Potts models attached to the complete graphs, the only spin models associated with symmetric conference graphs with
n ≥ 5 vertices are the pentagon and the lattice graph
L2(3) with 9 vertices. The proof avoids Jaeger's use of the classification of strongly regular graphs having strongly regular subconstituents due to P. J. Cameron, J. M. Goethals, and J. J. Seidel.
View full abstract