@inproceedings{lars1950conformal,
title={Conformal invariants and function-theoretic null-sets},
author={Lars Ahlfors, and Arne Beurling},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203128152129710},
booktitle={Acta Mathematica},
volume={83},
number={1},
pages={101-129},
year={1950},
}
Lars Ahlfors, and Arne Beurling. Conformal invariants and function-theoretic null-sets. 1950. Vol. 83. In Acta Mathematica. pp.101-129. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203128152129710.