@inproceedings{shing-tung1970on,
title={On the Fundamental Group of Manifolds of Non-positive Curvature},
author={Shing-Tung Yau},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191224205323029738294},
booktitle={Proceedings of the National Academy of Sciences},
volume={67},
number={2},
pages={509-509},
year={1970},
}
Shing-Tung Yau. On the Fundamental Group of Manifolds of Non-positive Curvature. 1970. Vol. 67. In Proceedings of the National Academy of Sciences. pp.509-509. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191224205323029738294.