An analogue of group divisible designs for Moore graphs

Abstract

We study graphs whose adjacency matrix S of order n satisfies the equation S + S 2 = J − K + kI, where J is a matrix of order n of all 1's, K is the direct sum on n l matrices of order l of all 1's, and I is the identity matrix. Moore graphs are the only solutions to the equation in the case l = 1 for which K = I. In the case k = l we can obtain Moore graphs from a solution S by a bordering process analogous to obtaining (ν, κ, λ)-designs from some group divisible designs. Other parameters are rare. We are able to find one new interesting graph with parameters k = 6, l = 4 on n = 40 vertices. We show that it has a transitive automorphism group isomorphic to C 4 × S 5.