@article{10_4153_CJM_1981_019_1,
     author = {Ahlbrandt, Calvin D. and Hinton, Don B. and Lewis, Roger T.},
     title = {Necessary and {Sufficient} {Conditions} for the {Discreteness} of the {Spectrum} of {Certain} {Singular} {Differential} {Operators}},
     journal = {Canadian journal of mathematics},
     pages = {229--246},
     year = {1981},
     volume = {33},
     number = {1},
     doi = {10.4153/CJM-1981-019-1},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1981-019-1/}
}