On uniqueness of Heine-Stieltjes polynomials for second order finite-difference equations

TL;DR

This paper proves that for second order finite-difference equations, at most one polynomial solution exists, extending classical continuous results and linking to the Bethe Ansatz, with implications for physical models.

Contribution

It broadens the conditions under which the uniqueness of polynomial solutions in the Heine-Stieltjes problem holds for difference equations.

Findings

At most one polynomial solution exists for the finite-difference equation.

The classical uniqueness results are extended under broader hypotheses.

The results relate to the nondegeneracy of solutions in physical models.

Abstract

A second order finite-difference equation has two linearly independent solutions. It is shown here that, like in the continuous case, at most one of the two can be a polynomial solution. The uniqueness in the classical continuous Heine-Stieltjes theory is shown to hold under broader hypotheses than usually presented. A difference between regularity condition and uniqueness is emphasized. Consistency of our uniqueness results is also checked against one of the Shapiro problems. An intrinsic relation between the Heine-Stieltjes problem and the discrete Bethe Ansatz equations allows one to immediately extend the uniqueness result from the former to the latter. The results have implications for nondegeneracy of polynomial solutions of physical models.