@inproceedings{nobuhiro1981a,
title={A classification of Busemann$G$-surfaces which possess convex functions},
author={Nobuhiro Innami},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203256441304311},
booktitle={Acta Mathematica},
volume={148},
number={1},
pages={15-29},
year={1981},
}
Nobuhiro Innami. A classification of Busemann$G$-surfaces which possess convex functions. 1981. Vol. 148. In Acta Mathematica. pp.15-29. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203256441304311.