@inproceedings{michael2014unicity,
title={Unicity of graded covers of the category $\mathcal{O}$ of Bernstein–Gelfand–Gelfand},
author={Michael Rottmaier, and Wolfgang Soergel},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203640928175086},
booktitle={Arkiv for Matematik},
volume={53},
number={2},
pages={383-400},
year={2014},
}
Michael Rottmaier, and Wolfgang Soergel. Unicity of graded covers of the category $\mathcal{O}$ of Bernstein–Gelfand–Gelfand. 2014. Vol. 53. In Arkiv for Matematik. pp.383-400. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203640928175086.