Remarks on pseudo-vertex-transitive graphs with small diameter

TL;DR

This paper investigates the property of pseudo-vertex transitivity in certain distance-regular graphs with small diameters, providing characterizations and examples for diameters 2, 3, and 4.

Contribution

It characterizes pseudo-vertex transitivity for diameter 2 graphs via local spectra and establishes that Taylor graphs (diameter 3) and antipodal tight graphs (diameter 4) are pseudo-vertex transitive.

Findings

Strongly regular graphs are pseudo-vertex-transitive iff all local graphs have the same spectrum.

Taylor graphs with diameter 3 are pseudo-vertex transitive.

Antipodal tight graphs with diameter 4 are pseudo-vertex transitive.

Abstract

Let $\Gamma$ denote a $Q$-polynomial distance-regular graph with vertex set $X$ and diameter $D$. Let $A$ denote the adjacency matrix of $\Gamma$. For a vertex $x\in X$ and for $0 \leq i \leq D$, let $E^*_i(x)$ denote the projection matrix to the $i$th subconstituent space of $\Gamma$ with respect to $x$. The Terwilliger algebra $T(x)$ of $\Gamma$ with respect to $x$ is the semisimple subalgebra of $\mathrm{Mat}_X(\mathbb{C})$ generated by $A, E^*_0(x), E^*_1(x), \ldots, E^*_D(x)$. Let $V$ denote a $\mathbb{C}$-vector space consisting of complex column vectors with rows indexed by $X$. We say $\Gamma$ is pseudo-vertex-transitive whenever for any vertices $x,y \in X$, there exists a $\mathbb{C}$-vector space isomorphism $\rho:V\to V$ such that $(\rho A - A \rho)V=0$ and $(\rho E^*_i(x) - E^*_i(y)\rho)V=0$ for all $0\leq i \leq D$. In this paper, we discuss pseudo-vertex transitivity for distance-regular graphs with diameter $D\in \{2,3,4\}$. For $D=2$, we show that a strongly regular graph is pseudo-vertex-transitive if and only if all its local graphs have the same spectrum. For $D = 3$, we consider the Taylor graphs and show that they are pseudo-vertex transitive. For $D=4$, we consider the antipodal tight graphs and show that they are pseudo-vertex transitive.