@inproceedings{henrik1991quadrature,
title={Quadrature surfaces as free boundaries},
author={Henrik Shahgholian},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203544593510634},
booktitle={Arkiv for Matematik},
volume={32},
number={2},
pages={475-492},
year={1991},
}