The Factorial-Basis Method for Finding Definite-Sum Solutions of Linear Recurrences With Polynomial Coefficients

Abstract

The problem of finding a nonzero solution of a linear recurrence $Ly=0$ with polynomial coefficients where $y$ has the form of a definite hypergeometric sum, related to the Inverse Creative Telescoping Problem of [14][Sec. 8], has now been open for three decades. Here we present an algorithm (implemented in a SageMath package) which, given such a recurrence and a quasi-triangular, shift-compatible factorial basis $\mathcal{B}=⟨P_k(n){⟩}_{k=0}^{\infty }$ of the polynomial space $\mathbb{K}[n]$ over a field $\mathbb{K}$ of characteristic zero, computes a recurrence satisfied by the coefficient sequence $c=⟨c_k{⟩}_{k=0}^{\infty }$ of the solution $y_n={\sum }_{k=0}^{\infty }{c}_k{P}_k(n)$ (where, thanks to the quasi-triangularity of $\mathcal{B}$, the sum on the right terminates for each $n\in \mathbb{N}$). More generally, if $\mathcal{B}$ is $m$-sieved for some $m\in \mathbb{N}$, our algorithm computes a system of $m$ recurrences satisfied by the $m$-sections of the coefficient sequence $c$. If an explicit nonzero solution of this system can be found, we obtain an explicit nonzero solution of $Ly=0$.

arXiv comment62 pages

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