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Inverse problems in the class of Q-polynomial graphs
I. N. Belousovab, A. A. Makhnevab 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:
In the class of distance-regular graphs $\Gamma$ of diameter 3 with a pseudogeometric graph $\Gamma_3$, feasible intersection arrays for the partial geometry were found for networks by Makhnev, Golubyatnikov, and Guo; for dual networks by Belousov and Makhnev; and for generalized quadrangles by Makhnev and Nirova. These authors obtained four infinite series of feasible intersection arrays of distance-regular graphs: $$\big\{c_2(u^2-m^2)+2c_2m-c_2-1,c_2(u^2-m^2),\ (c_2-1)(u^2-m^2)+2c_2m-c_2;1,c_2,u^2-m^2\big\},$$ $$\{mt,(t+1)(m-1),t+1;1,1,(m-1)t\}\ \ \text{for}\ \ m\le t,$$ $$\{lt,(t-1)(l-1),t+1;1,t-1,(l-1)t\},\ \ \text{and}\ \ \{a(p+1),ap,a+1;1,a,ap\}.$$ We find all feasible intersection arrays of $Q$-polynomial graphs from these series. In particular, we show that, among these infinite families of feasible arrays, only two arrays ($\{7,6,5;1,2,3\}$ (folded 7-cube) and $\{191,156,153;1,4,39\}$) correspond to $Q$-polynomial graphs.
Keywords:
distance-regular graph, $Q$-polynomial graph, graph $\Gamma$ with a strongly regular graph $\Gamma_3$.
Received: 22.05.2020 Revised: 17.06.2020 Accepted: 13.07.2020
Citation:
I. N. Belousov, A. A. Makhnev, “Inverse problems in the class of Q-polynomial graphs”, Trudy Inst. Mat. i Mekh. UrO RAN, 26, no. 3, 2020, 14–22
Linking options:
https://www.mathnet.ru/eng/timm1741 https://www.mathnet.ru/eng/timm/v26/i3/p14
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Abstract page: | 132 | Full-text PDF : | 38 | References: | 32 | First page: | 6 |
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