@inproceedings{kari1993area,
title={Area distortion of quasiconformal mappings},
author={Kari Astala},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203322682989501},
booktitle={Acta Mathematica},
volume={173},
number={1},
pages={37-60},
year={1993},
}