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 $\Gamma_3$.
UDC:
519.17
MSC: 05C25 Received: 22.05.2020
Revised: 17.06.2020
Accepted: 13.07.2020
DOI:
10.21538/0134-4889-2020-26-3-14-22