An ubiquitous three-term recurrence relation.

We solve an eigenvalue equation that appears in several papers about a wide range of physical problems. The Frobenius method leads to a three-term recurrence relation for the coefficients of the power series that, under suitable truncation, yields exact analytical eigenvalues and eigenfunctions for particular values of a model parameter. From these solutions, some researchers have derived a variety of predictions such as allowed angular frequencies, allowed field intensities, and the like. We also solve the eigenvalue equation numerically by means of the variational Rayleigh–Ritz method and compare the resulting eigenvalues with those provided by the truncation condition. In this way, we prove that those physical predictions are merely artifacts of the truncation condition. [ABSTRACT FROM AUTHOR]