@inproceedings{edgar1969a,
title={A minimax inequality for operators and a related numerical range},
author={Edgar Asplund, and Vlastimil Pták},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203221253368105},
booktitle={Acta Mathematica},
volume={126},
number={1},
pages={53-62},
year={1969},
}
Edgar Asplund, and Vlastimil Pták. A minimax inequality for operators and a related numerical range. 1969. Vol. 126. In Acta Mathematica. pp.53-62. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203221253368105.