A generalization of the Askey-Wilson relations using a projective geometry

Abstract

In this paper, we present a generalization of the Askey-Wilson relations that involves a projective geometry. A projective geometry is defined as follows. Let $h>k\geq 1$ denote integers. Let $\mathbb{F}_{q}$ denote a finite field with $q$ elements. Let $\mathcal{V}$ denote an $(h+k)$-dimensional vector space over $\mathbb{F}_{q}$. Let the set $P$ consist of the subspaces of $\mathcal{V}$. The set $P$, together with the inclusion partial order, is a poset called a projective geometry. We define a matrix $A\in \text{Mat}_{P}(\mathbb{C})$ as follows. For $u,v\in P$, the $(u,v)$-entry of $A$ is $1$ if each of $u,v$ covers $u\cap v$, and $0$ otherwise. Fix $y\in P$ with $\dim y=k$. We define a diagonal matrix $A^*\in \text{Mat}_{P}(\mathbb{C})$ as follows. For $u\in P$, the $(u,u)$-entry of $A^{*}$ is $q^{\dim(u\cap y)}$. We show that \begin{align*} &A^2A^{*}-\bigl(q+q^{-1}\bigr)AA^{*}A+A^{*}A^{2}-\mathcal{Y}\bigl(AA^{*}+A^{*}A\bigr)-\mathcal{P} A^{*}=ΩA+G, \newline &A^{*2}A-\bigl(q+q^{-1}\bigr) A^*AA^*+AA^{*2}=\mathcal{Y}A^{*2}+ΩA^{*}+G^{*}, \end{align*} where $\mathcal{Y}, \mathcal{P}, Ω, G, G^*$ are matrices in $\text{Mat}_{P}(\mathbb{C})$ that commute with each of $A, A^*$. We give precise formulas for $\mathcal{Y}, \mathcal{P}, Ω, G, G^*$.