Fixed-Lowering $\mathfrak{sl}_2$-Triples for Laurent-Shift Operators: Exact Stencil Endpoints and Recurrence Locality

Abstract

Let $S$ be the unit forward shift, and let $$ \mathcal{A} = \mathbb{K}[x]\langle S,S^{-1}\rangle \big/ \bigl(Sx-(x+1)S\bigr) $$ be the algebra of finite Laurent-shift operators with polynomial coefficients over a characteristic-zero field $\mathbb{K}$. We classify all $\mathfrak{sl}_2$-triples in $\mathcal{A}$ with fixed lowering operator $F=1-S$. Writing $D=S-1$ and $X=xS^{-1}$, every completion is uniquely determined by $λ\in\mathbb{K}$ and $g\in\mathbb{K}[S,S^{-1}]$, with $$ H=2(X+g)D-λ,\qquad E=(X+g)^2D-λ(X+g). $$ We give an intrinsic recognition and reconstruction from $H$ and determine the exact extreme shifts of $H$ and $E$. The associated monic eigenpolynomials form a $Δ$-Appell sequence. Multiplication by $x$ has finite lower recurrence bandwidth exactly when $g\in S^{-1}\mathbb{K}[S]$; in this case, the bandwidth equals the right endpoint of the Cartan stencil. Otherwise, $H$ and $E$ remain finite-order, while the degree recurrence has an infinite tail whose eventual signed coefficients form a polynomial recovering the lowest Laurent term of $g$. Over $\mathbb{R}$, positive-measure orthogonality occurs exactly for translated monic Charlier systems.