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arXiv · 2505.21272

Quasi-strongly regular graphs on the flags of symmetric designs

Eugenia O'Reilly-Regueiro·Octavio B. Zapata-Fonseca
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Abstract

This paper was inspired by a paper by Blokhuis and Brouwer [Designs, Codes and Cryptography 65, 2012] in which a definition of a graph on the flags of a biplane is given, and they prove that the graph corresponding to the unique $(11,5,2)$-biplane is determined by its spectrum. It is also inspired by the different definition of flag-graph seen in the context of maps and abstract polytopes. Here we use this definition for $(v,k,\lambda)$-BIBDs, and prove that if the design is symmetric then the graph is quasi-strongly regular. We will also use the definition given by Blokhuis and Brouwer for the case of biplanes and prove that this too, is a QSRG, (with different parameters). We investigate whether these graphs are determined by their spectra for some of the known biplanes.

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Eugenia O'Reilly-Regueiro, Octavio B. Zapata-Fonseca. 2025-05-27. Quasi-strongly regular graphs on the flags of symmetric designs. https://arxiv.org/abs/2505.21272

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