@article{10_1017_S0017089500000562,
     author = {Bonsall, F. F.},
     title = {The numerical range of an element of a normed algebra},
     journal = {Glasgow mathematical journal},
     pages = {68--72},
     year = {1969},
     volume = {10},
     number = {1},
     doi = {10.1017/S0017089500000562},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500000562/}
}