@article{10_4153_CJM_2003_031_0,
     author = {Cao, Xifang and Kong, Qingkai and Wu, Hongyou and Zettl, Anton},
     title = {Sturm-Liouville {Problems} {Whose} {Leading} {Coefficient} {Function} {Changes} {Sign}},
     journal = {Canadian journal of mathematics},
     pages = {724--749},
     year = {2003},
     volume = {55},
     number = {4},
     doi = {10.4153/CJM-2003-031-0},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-2003-031-0/}
}