@inproceedings{wolfhard1992a,
title={A converse to the mean value theorem for harmonic functions},
author={Wolfhard Hansen, and Nikolai Nadirashvili},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203320635114488},
booktitle={Acta Mathematica},
volume={171},
number={2},
pages={139-163},
year={1992},
}
Wolfhard Hansen, and Nikolai Nadirashvili. A converse to the mean value theorem for harmonic functions. 1992. Vol. 171. In Acta Mathematica. pp.139-163. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203320635114488.