@inproceedings{hwa-long2008invertibility,
title={Invertibility and Fredholmness of linear combinations of quadratic, k-potent and nilpotent operators},
author={Hwa-Long Gau, Chih-Jen Wang, and Ngai-Ching Wong},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020205323191805173},
booktitle={Operators and Matrices},
volume={2},
number={2},
pages={193-199},
year={2008},
}

Hwa-Long Gau, Chih-Jen Wang, and Ngai-Ching Wong. Invertibility and Fredholmness of linear combinations of quadratic, k-potent and nilpotent operators. 2008. Vol. 2. In Operators and Matrices. pp.193-199. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020205323191805173.