A NOTE ON STRONGLY REGULAR GRAPHS AND (k, τ)-REGULAR
SETS∗
PAULA CARVALHO†
Abstract. A subset of the vertex set of a graph G, S ⊆ V (G), is a (k, τ)-regular set if it induces
a k-regular subgraph of G and every vertex not in the subset has τ neighbors in it. This paper
is a contribution to the given problem of existence of (k, τ)-regular sets associated with all distinct
eigenvalues of integral strongly regular graphs. The minimal idempotents of the Bose-Mesner algebra
of strongly regular graphs are used to obtain a necessary and sufficient condition on the existence of
(k, τ)-regular sets for its two restricted eigenvalues.
Key words. Graph theory; Graph spectra; Integral graphs; Strongly regular graphs; Dominating
sets.