The $\mathrm{U}$-Derivative Associated with Linear Recurrence Sequences

Abstract

We associate a canonical operator to every linear recurrence sequence admitting a distinct-root Binet representation \(\mathrm{U}_n=\sum_{j=1}^{k}A_jλ_j^n.\) The construction is based on the spectral Binet operator \(\mathcal{B}_\mathrm{U}=\sum_{j=1}^{k}A_jT_{λ_j}, \) where \(T_{λ_j}f(x)=f(λ_jx)\) is the dilation operator associated with the characteristic root \(λ_j\). The corresponding recurrence derivative is defined by \(\mathrm{D}_\mathrm{U}=x^{-1}\mathcal{B}_\mathrm{U}.\) Its fundamental property is the monomial identity \(\mathrm{D}_\mathrm{U}(x^n)=\mathrm{U}_nx^{n-1},\) which establishes a direct correspondence between recurrence sequences and degree-lowering operators. We develop the operator theory of \(\mathrm{D}_\mathrm{U}\), including iteration, a spectral product rule, the associated Leibniz defect, and a quotient rule. The construction reveals a natural bifurcation determined by the value \(\mathrm{U}_0\), producing the Ward and non-Ward regimes as intrinsic operatorial phenomena. The product structure is shown to be governed by \(k-2\) residual spectral channels, leading to notions of spectral cancellation and spectral balance. As an application, we obtain a complete qualitative classification of second-order recurrence calculi. Using Horadam sequences as a universal second-order model, we prove that every such calculus is determined by the invariant triple \((\mathrm{W}_0,Δ, Z_\mathrm{W}),\) where \(\mathrm{W}_0\) identifies the Ward/non-Ward type, \(Δ\) determines the spectral geometry, and \(Z_\mathrm{W}\) determines regularity or singularity.