The Terwilliger polynomial of a Q-polynomial distance-regular graph and its application to the pseudo-partition graphs

Abstract

Let $\Gamma$ be a $Q$-polynomial distance-regular graph with diameter at least $3$. Terwilliger (1993) implicitly showed that there exists a polynomial, say $T(\lambda)\in \mathbb{C}[\lambda]$, of degree $4$ depending only on the intersection numbers of $\Gamma$ and such that $T(\eta)\geq 0$ holds for any non-principal eigenvalue $\eta$ of the local graph $\Gamma(x)$ for any vertex $x\in V(\Gamma)$. We call $T(\lambda)$ the Terwilliger polynomial of $\Gamma$. In this paper, we give an explicit formula for $T(\lambda)$ in terms of the intersection numbers of $\Gamma$ and its dual eigenvalues. We then apply this polynomial to show that all pseudo-partition graphs with diameter at least $3$ are known.