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 \(\textsf{Sp}(2d,q)\). 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 \(\textsf{Sp}(2d,q)\). Furthermore, a generalization of the construction is discussed.
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Acknowledgements
We would like to thank the referees for their valuable remarks.The research of Alexander Gavrilyuk is supported by JSPS KAKENHI Grant Number 22K03403.
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Gavrilyuk, A.L., Kabanov, V.V. Strongly regular graphs decomposable into a divisible design graph and a Hoffman coclique. Des. Codes Cryptogr. (2023). https://doi.org/10.1007/s10623-023-01348-9
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DOI: https://doi.org/10.1007/s10623-023-01348-9