@inproceedings{olof1952retraction,
title={Retraction and extension of mappings of metric and nonmetric spaces},
author={Olof Hanner},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203416159733886},
booktitle={Arkiv for Matematik},
volume={2},
number={4},
pages={315-360},
year={1952},
}
Olof Hanner. Retraction and extension of mappings of metric and nonmetric spaces. 1952. Vol. 2. In Arkiv for Matematik. pp.315-360. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203416159733886.