ABSTRACT
NN> has zero as an eigenvalue for k > r. For a generalization to m associate class PBIBDs, see [1663, 1770].
7.89 Proposition Let N be the incidence matrix of a group divisible PBIBD(v, k, λ1, λ2) with m groups of size n. The eigenvalues of NN> are rk, with multiplicity 1; r− λ1, with multiplicity m − 1; and rk − vλ2, with multiplicity n(m − 1).
