@inproceedings{a.1954on,
title={On indefinite binary quadratic forms},
author={A. Oppenheim},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203138832092788},
booktitle={Acta Mathematica},
volume={91},
number={1},
pages={43-50},
year={1954},
}