S. Pruess et al., AN ASYMPTOTIC NUMERICAL-METHOD FOR A CLASS OF SINGULAR STURM LIOUVILLE PROBLEMS/, SIAM journal on numerical analysis, 32(5), 1995, pp. 1658-1676
A class of Sturm-Liouville problems has been discovered for which the
method of approximating the coefficients by piecewise constant functio
ns has considerable difficulty. We characterize this class, give examp
les, and derive a modification of the method based on local asymptotic
approximations which surmounts the difficulties.