@inproceedings{lars1978two,
title={Two approximation problems in function spaces},
author={Lars Inge Hedberg},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203502285629280},
booktitle={Arkiv for Matematik},
volume={16},
number={1},
pages={51-81},
year={1978},
}