@article{10_1017_S0017089500000896,
     author = {Crabb, M. J.},
     title = {Numerical range estimates for the norms of iterated operators},
     journal = {Glasgow mathematical journal},
     pages = {85--87},
     year = {1970},
     volume = {11},
     number = {2},
     doi = {10.1017/S0017089500000896},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500000896/}
}