English

On the classification of triply-transitive strongly-regular graphs

Combinatorics 2025-07-22 v1 Rings and Algebras

Abstract

Let Γ=(Ω,E)\Gamma = (\Omega,E) be a strongly-regular graph with adjacency matrix A1A_1, and let A2A_2 be the adjacency matrix of its complement. For any vertex ωΩ\omega\in \Omega, we define E0,ωE_{0,\omega}^* E1,ωE_{1,\omega}^* and E2,ωE_{2,\omega}^* to be respectively the diagonal matrices whose main diagonal is the row corresponding to ω\omega in the matrices I,A1I, A_1, and A2A_2. The Terwilliger algebra of Γ\Gamma with respect to the vertex ωΩ\omega\in \Omega is the subalgebra Tω=I,A1,A2,E0,ω,E1,ω,E2,ωT_\omega = \left\langle I,A_1,A_2,E_{0,\omega}^*,E_{1,\omega}^*,E_{2,\omega}^* \right\rangle of the complex matrix algebra MΩ(C)\operatorname{M_{|\Omega|}}(\mathbb{C}). The algebra TωT_\omega contains the subspace T0,ω=Span{Ei,ωAjEk,ω:0i,j,k2}T_{0,\omega} = \operatorname{Span}\left\{ E_{i,\omega}^*A_jE_{k,\omega}^*: 0\leq i,j,k\leq 2 \right\}. In addition, if G=\AutΓG = \Aut{\Gamma}, then TωT_\omega is a subalgebra of the centralizer algebra T~ω=\EndGωCΩ\tilde{T}_\omega = \End{G_\omega}{\mathbb{C}^\Omega}. The strongly-regular graph Γ=(Ω,E)\Gamma=(\Omega,E) is triply transitive if Γ\Gamma is vertex transitive and T0,ω=Tω=T~ωT_{0,\omega} = T_\omega = \tilde{T}_\omega, for any ωΩ\omega \in \Omega. In this paper, we classify all triply transitive strongly-regular graphs that are not isomorphic to the collinearity graph of the polar space O6(q)O_{6}^-(q), where qq is a prime power, or the affine polar graph \vo2mε(2)\vo_{2m}^\varepsilon(2), where m1m\geq 1 and ε=±1\varepsilon = \pm 1.

Keywords

Cite

@article{arxiv.2507.14320,
  title  = {On the classification of triply-transitive strongly-regular graphs},
  author = {Allen Herman and Roghayeh Maleki and Andriaherimanana Sarobidy Razafimahatratra},
  journal= {arXiv preprint arXiv:2507.14320},
  year   = {2025}
}

Comments

48 pages

R2 v1 2026-07-01T04:08:41.071Z