@article{JRAM_1969__237_150949,
     author = {John A R. Holbrook},
     title = {Multiplicative properties of the numerical radius in operator theory.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {166--174},
     publisher = {mathdoc},
     volume = {237},
     year = {1969},
     zbl = {0176.43502},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1969__237_150949/}
}