This paper presents a method to determine whether the second-order linear differential equation 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.
"Convergent Disfocality and Nondisfocality Criteria for Second-Order Linear Differential Equations." Abstr. Appl. Anal. 2013 1 - 11, 2013. https://doi.org/10.1155/2013/987976