Strongly regular n-e.c. graphs

A result of Erdös and Rényi shows that for a fixed integer n almost all graphs satisfy the n-e.c. adjacency property. However, there are few explicit constructions of n e.c. graphs for n > 2, and almost all known families of n-e.c. graphs are strongly regular graphs. In this paper we derive parameter bounds on strongly regular n-e.c. graphs constructed from the point sets of partial geometries. This work generalizes bounds on n-e.c. block intersection graphs of balanced incomplete block designs given by McKay and Pike. It also relates to work by Griggs, Grannel, and Forbes' determining 3-e.c. graphs that are block intersection graphs of Steiner triple systems. In addition to these bounds, we give examples of strongly regular graphs that contain every possible subgraph of small order but are not n-e.c. for n > 2.

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