Some $q$-exponential formulas involving the double lowering operator $ψ$ for a tridiagonal pair

Abstract

Let $\mathbb{K}$ denote an algebraically closed field and let $V$ denote a vector space over $\mathbb{K}$ with finite positive dimension. Let $A,A^*$ denote a tridiagonal pair on $V$. We assume that $A,A^*$ belongs to a family of tridiagonal pairs said to have $q$-Racah type. Let $\{U_i\}_{i=0}^d$ and $\{U_i^\Downarrow\}_{i=0}^{d}$ denote the first and second split decompositions of $V$. In an earlier paper we introduced a double lowering operator $ψ:V\to V$ with the notable feature that both $ψU_i\subseteq U_{i-1}$ and $ψU_i^\Downarrow\subseteq U_{i-1}^\Downarrow$ for $0\leq i\leq d$, where $U_{-1}=0$ and $U_{-1}^\Downarrow=0$. In the same paper, we showed that there exists a unique linear transformation $Δ:V\to V$ such that $Δ(U_i)\subseteq U_i^{\Downarrow}$ and $(Δ-I)U_i\subseteq U_0+U_1+\cdots +U_{i-1}$ for $0\leq i \leq d$. In the present paper, we show that $Δ$ can be expressed as a product of two linear transformations; one is a $q$-exponential in $ψ$ and the other is a $q^{-1}$-exponential in $ψ$. We view $Δ$ as a transition matrix from the first split decomposition of $V$ to the second. Consequently, we view the $q^{-1}$-exponential in $ψ$ as a transition matrix from the first split decomposition to a decomposition of $V$ which we interpret as a kind of halfway point. This halfway point turns out to be the eigenspace decomposition of a certain linear transformation $\mathcal{M}$. We discuss the eigenspace decomposition of $\mathcal{M}$ and give the actions of various operators on this decomposition.