@inproceedings{h.1962the,
title={The fundamental group of a surface, and a theorem of Schreier},
author={H. B. Griffiths},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203201884779958},
booktitle={Acta Mathematica},
volume={110},
number={1},
pages={1-17},
year={1962},
}

H. B. Griffiths. The fundamental group of a surface, and a theorem of Schreier. 1962. Vol. 110. In Acta Mathematica. pp.1-17. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203201884779958.