@article{10_1017_S0017089500006443,
     author = {Aparicio, C. and Oca\~na, F. and Pay\'a, R. and Rodr{\'\i}guez, A.},
     title = {A non-smooth extension of {Frechet} differentiability of the norm with applications to numerical ranges},
     journal = {Glasgow mathematical journal},
     pages = {121--137},
     year = {1986},
     volume = {28},
     number = {2},
     doi = {10.1017/S0017089500006443},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500006443/}
}