@article{10_1017_S001708950000447X,
     author = {Barra, G. de},
     title = {The boundary of the numerical range},
     journal = {Glasgow mathematical journal},
     pages = {69--72},
     year = {1981},
     volume = {22},
     number = {1},
     doi = {10.1017/S001708950000447X},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S001708950000447X/}
}