Convergent disfocality and nondisfocality criteria for second-order linear differential equations (Q2319347)

Summary: This paper presents a method to determine whether the second-order linear differential equation \(y'' + q(x) y = 0\) is either disfocal or nondisfocal in a fixed interval. The method is based on the recursive application of a linear operator to certain functions and yields upper and lower bounds for the distances between a zero and its adjacent critical points, which will be shown to converge to the exact values of such distances as the recursivity index grows.