@inproceedings{m.1970some,
title={Some extremal problems in the theory of numerical ranges},
author={M. J. Crabb, J. Duncan, and C. M. McGregor},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203224565218128},
booktitle={Acta Mathematica},
volume={128},
number={1},
pages={123-142},
year={1970},
}
M. J. Crabb, J. Duncan, and C. M. McGregor. Some extremal problems in the theory of numerical ranges. 1970. Vol. 128. In Acta Mathematica. pp.123-142. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203224565218128.