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For example, the pentagon is strongly regular with parameters (v, k, λ, μ) = (5, 2, 0, 1). One easily verifies that a graph G is strongly regular with parameters (v, k, λ, μ) if and only if its complement G is strongly regular with parameters (v, v−k− 1, v− 2k+μ− 2, v−2k+λ). The line graph of the complete graph of order m, known as the triangular graph T (m), is strongly regular with parameters ( 2 m(m− 1), 2(m− 2),m− 2, 4). The complement of T (5) has parameters (10, 3, 0, 1). This is the Petersen graph. A graph G satisfying condition (i) is called k-regular. It is well-known and easily seen that the adjacency matrix of a k-regular graph has an eigenvalue k with eigenvector 1 (the all-one vector), and that every other eigenvalue ρ satisfies |ρ| ≤ k (see Biggs [4]). For convenience we call an eigenvalue restricted if it has an eigenvector perpendicular to 1. We let I and J denote the identity and all-one matrices, respectively.