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On distance-regular graphs with intersection arrays $\{q^2-1,q(q-2),q+2;1,q,(q+1)(q-2)\}$
A. A. Makhnevab, D. V. Paduchikha a Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg
b Ural Federal University named after the First President of Russia B. N. Yeltsin, Ekaterinburg
Abstract:
If a distance-regular graph $\Gamma$ of diameter 3 contains a maximal locally regular 1-code that is last subconstituent perfect, then $\Gamma$ has intersection array $\{a(p+1),cp,a+1;1,c,ap\}$ or $\{a(p+1),(a+1)p,c;1,c,ap\}$, where $a=a_3$, $c=c_2$, and $p=p^3_{33}$ (Jurišić, Vidali). In the first case, $\Gamma$ has eigenvalue $\theta_2=-1$ and the graph $\Gamma_3$ is pseudogeometric for $GQ(p+1,a)$. If $a=c+1$, then the graph $\bar\Gamma_2$ is pseudogeometric for $pG_2(p+1,2a)$. If in this case the pseudogeometric graph for the generalized quadrangle $GQ(p+1,a)$ has quasi-classical parameters, then $\Gamma$ has intersection array $\{q^2-1,q(q-2),q+2;1,q,(q+1)(q-2)\}$ (Makhnev, Nirova). In this paper, we find possible automorphisms of a graph with intersection array $\{q^2-1,q(q-2),q+2;1,q,(q+1)(q-2)\}$.
Keywords:
distance-regular graph, generalized quadrangle, graph automorphism.
Received: 10.09.2020 Revised: 20.12.2020 Accepted: 11.01.2021
Citation:
A. A. Makhnev, D. V. Paduchikh, “On distance-regular graphs with intersection arrays $\{q^2-1,q(q-2),q+2;1,q,(q+1)(q-2)\}$”, Trudy Inst. Mat. i Mekh. UrO RAN, 27, no. 1, 2021, 146–156
Linking options:
https://www.mathnet.ru/eng/timm1799 https://www.mathnet.ru/eng/timm/v27/i1/p146
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