@inproceedings{pavel2005an,
title={An isoperimetric problem for point interactions},
author={Pavel Exner},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020130946391683662},
booktitle={Journal of Physics A: Mathematical and General},
volume={38},
number={22},
pages={4795},
year={2005},
}

Pavel Exner. An isoperimetric problem for point interactions. 2005. Vol. 38. In Journal of Physics A: Mathematical and General. pp.4795. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020130946391683662.