@inproceedings{einar1952behavior,
title={Behavior of solutions of linear second order differential equations},
author={Einar Hille},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203414120554872},
booktitle={Arkiv for Matematik},
volume={2},
number={1},
pages={25-41},
year={1952},
}
Einar Hille. Behavior of solutions of linear second order differential equations. 1952. Vol. 2. In Arkiv for Matematik. pp.25-41. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203414120554872.