The Terwilliger algebra of the Odd graph revisited from the viewpoint of group action

Abstract

Let $O_{m+1}$ denote the Odd graph on a set of cardinality $2m+1$, where $m$ is a positive integer. Denote by $X$ its vertex set and by $T:=T(x_0)$ its Terwilliger algebra with respect to any fixed vertex $x_0\in X$. In this paper, we first prove that $T$ coincides with the centralizer algebra of the stabilizer of $x_0$ in the automorphism group of $O_{m+1}$ by considering the action of this automorphism group on $X\times X\times X$. Then we give the decomposition of $T$ for $m\geq 3$ by using all the homogeneous components of $V:=\mathbb{C}^X$, each of which is a nonzero subspace of $V$ spanned by the irreducible $T$-modules that are isomorphic. Finally, we display an orthogonal basis for every homogeneous component of $V$.