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JOURNALS // Matematicheskie Trudy // Archive

Mat. Tr., 2022 Volume 25, Number 2, Pages 162–173 (Mi mt673)

On distance-regular graphs of diameter $3$ with eigenvalue $0$

A. A. Makhnevab, I. N. Belousovba

a Ural Federal University named after the First President of Russia B. N. Yeltsin, Ekaterinburg
b N.N. Krasovskii Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg

Abstract: Graph $\Gamma_i$ for a distance-regular graph $\Gamma$ of diameter $3$ can be strongly regular for $i=2$ or $i=3$. J. Koolen with coauthors found parameters of $\Gamma_2$ by the intersection array of graph $\Gamma$ (independently parameters were obtained by Makhnev A.A. and Paduchikh D.V.). In this case $\Gamma$ has eigenvalue $\theta=a_2-c_3$. In this paper it is consider graphs with eigenvalues $\theta_2=0$ and $\theta_3=a_2-c_3$. It is proved that $\Gamma$ has intersection array $\{yx+yz,yz-y,xy-x;1,x+z,yz\}$. Further if $a_2-c_3\ge -10$ then $\Gamma$ has intersection array $\{12,6,2;1,4,9\}$, $\{60,45,8;1,12,50\}$, $\{63,42,12;1,9,49\}$ or $\{72,45,16; 1,8,54\}$.

Key words: strongly regular graph, distance-regular graph, eigenvalue.

UDC: УДК 519.17

Received: 21.01.2022
Revised: 10.09.2022
Accepted: 02.11.2022

DOI: 10.33048/mattrudy.2022.25.207


 English version:
Siberian Advances in Mathematics, 2023, 33:1, 56–65


© Steklov Math. Inst. of RAS, 2025