@article{10_1017_S0017089500006315,
     author = {Crabb, M. J. and McGregor, C. M.},
     title = {Numerical ranges of powers of hermitian elements},
     journal = {Glasgow mathematical journal},
     pages = {37--45},
     year = {1986},
     volume = {28},
     number = {1},
     doi = {10.1017/S0017089500006315},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500006315/}
}