@inproceedings{fei2010gravitational,
title={Gravitational anomaly cancellation and modular invariance},
author={Fei Han, and Kefeng Liu},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191221111612903104670},
booktitle={arXiv preprint arXiv:1007.5295},
year={2010},
}