@article{10_1017_S0017089508004564,
     author = {LI, CHI-KWONG and POON, YIU-TUNG},
     title = {SPECTRUM, {NUMERICAL} {RANGE} {AND} {DAVIS-WIELANDT} {SHELL} {OF} {A} {NORMAL} {OPERATOR}},
     journal = {Glasgow mathematical journal},
     pages = {91--100},
     year = {2009},
     volume = {51},
     number = {1},
     doi = {10.1017/S0017089508004564},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089508004564/}
}