@inproceedings{chi-kwong2015the,
title={The spectrum of the product of operators, and the product of their numerical ranges},
author={Chi-Kwong Li, Ming-Cheng Tsai, Kuo-Zhong Wang, and Ngai-Ching Wong},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020210142554097199},
booktitle={Linear Algebra and its Applications},
volume={469},
pages={487-499},
year={2015},
}

Chi-Kwong Li, Ming-Cheng Tsai, Kuo-Zhong Wang, and Ngai-Ching Wong. The spectrum of the product of operators, and the product of their numerical ranges. 2015. Vol. 469. In Linear Algebra and its Applications. pp.487-499. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020210142554097199.