@inproceedings{a.1940abstract,
title={Abstract flat projective differential geometry},
author={A. D. Michal, and A. B. Mewborn},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203116194175619},
booktitle={Acta Mathematica},
volume={72},
number={1},
pages={259-281},
year={1940},
}