The vertices of
may be identified with the 2-subsets of that are adjacent iff the
2-subsets have a nonempty intersection (Ball and Coxeter 1987, p. 304; Brualdi
and Ryser 1991, p. 152), namely the Johnson graph .
Chang (1959, 1960) and Hoffman (1960) showed that if is a strongly regular
graph on the parameters with , then if , is isomorphic to the triangular graph . If , then is isomorphic to one of three graphs known as the Chang
graphs or to
(Brualdi and Ryser 1991, p. 152).
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Matrix Theory. New York: Cambridge University Press, p. 152, 1991.Chang,
L.-C. "The Uniqueness and Non-Uniqueness of the Triangular Association Scheme."
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12-18, 1960.Hoffman, A. J. "On the Uniqueness of the Triangular
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