Strongly regular graphs decomposable into a divisible design graph and a Hoffman coclique

Abstract: In 2022, the second author found a prolific construction of strongly regular graphs, which is based on joining a coclique and a divisible design graph with certain parameters. The construction produces strongly regular graphs with the same parameters as the complement of the symplectic graph $\mathsf{Sp}(2d,q)$. In this paper, we determine the parameters of strongly regular graphs which admit a decomposition into a divisible design graph and a coclique attaining the Hoffman bound. In particular, it is shown that when the least eigenvalue of such a strongly regular graph is a prime power, its parameters coincide with those of the complement of $\mathsf{Sp}(2d,q)$. Furthermore, a generalization of the construction is discussed.