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Trudy Instituta Matematiki i Mekhaniki UrO RAN, 2019, Volume 25, Number 4, Pages 136–141
DOI: https://doi.org/10.21538/0134-4889-2019-25-4-136-141
(Mi timm1678)
 

Nonexistence of certain Q-polynomial distance-regular graphs

A. A. Makhnevab, M. P. Golubyatnikovab

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
References:
Abstract: I. N. Belousov, A. A. Makhnev, and M. S. Nirova described $Q$-polynomial distance-regular graphs $\Gamma$ of diameter 3 for which the graphs $\Gamma_2$ and $\Gamma_3$ are strongly regular. Set $a=a_3$. A graph $\Gamma$ has type (I) if $c_2+1$ divides $a$, type (II) if $c_2+1$ divides $a+1$, and type (III) if $c_2+1$ divides neither $a$ nor $a+1$. If $\Gamma$ is a graph of type (II), then $a+1=w(c_2+1)$, $t^2=w(w(c_2+1)+c_2)$, and either (i) $w=s^2$, $t^2=s^2(s^2(c_2+1)+c_2)$, $(s^2(c_2+1)+c_2$ is the square of an integer $u$, $c_2=(u^2-s^2)/(s^2+1)$, $t=su$, and $a=(u^2s^2-1)/(s^2+1)$ or (ii) $c_2=sw$, $t^2=w^2(sw+1+s)$, $sw+1+s$ is the square of an integer $u$, $c_2=(u^2-1)w/(w+1)$, $t=uw$, $a=(u^2w^2-1)/(w+1)$, and $\Gamma$ has intersection array
$$\left\{ \frac{u^3w^2+u^2w^2+uw-1}{w+1},\frac{(u^2-1)uw^2}{w+1},\frac{(u^2w+1)w}{w+1};1,\frac{(u^2-1)w}{w+1},\frac{(u^2w+1)uw}{w+1}\right\}.$$
If a graph of type (IIii) is such that $w=u$, then it has intersection array $\{w^4+w-1,w^4-w^3,(w^2-w+1)w;$ $1,w(w-1),(w^2-w+1)w^2\}$. We prove that graphs with such intersection arrays do not exist for even $w$.
Keywords: distance-regular graph, $Q$-polynomial graph.
Funding agency Grant number
Ministry of Education and Science of the Russian Federation 02.A03.21.0006
This work was supported by the Russian Academic Excellence Project (agreement no. 02.A03.21.0006 of August 27, 2013, between the Ministry of Education and Science of the Russian Federation and Ural Federal University).
Received: 10.09.2019
Revised: 07.11.2019
Accepted: 11.11.2019
Bibliographic databases:
Document Type: Article
UDC: 519.17
MSC: 05C25
Language: Russian
Citation: A. A. Makhnev, M. P. Golubyatnikov, “Nonexistence of certain Q-polynomial distance-regular graphs”, Trudy Inst. Mat. i Mekh. UrO RAN, 25, no. 4, 2019, 136–141
Citation in format AMSBIB
\Bibitem{MakGol19}
\by A.~A.~Makhnev, M.~P.~Golubyatnikov
\paper Nonexistence of certain Q-polynomial distance-regular graphs
\serial Trudy Inst. Mat. i Mekh. UrO RAN
\yr 2019
\vol 25
\issue 4
\pages 136--141
\mathnet{http://mi.mathnet.ru/timm1678}
\crossref{https://doi.org/10.21538/0134-4889-2019-25-4-136-141}
\elib{https://elibrary.ru/item.asp?id=41455529}
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