Spectra of products and numerical ranges. (English) Zbl 0143.36703

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[1] Berberian, S. K., The numerical range of a normal operator, Duke Math. J.,, 31, 479-483 (1964) · Zbl 0132.35702
[2] Day, M. M., Normed Linear Spaces (1962), Springer-Verlag: Springer-Verlag Berlin · Zbl 0109.33601
[3] Bishop, E.; Phelps, R. R., A proof that every Banach space is subreflexive, Bull. Amer. Math. Soc.,, 67, 97-98 (1961) · Zbl 0098.07905
[4] Lumer, G., Semi-inner-product spaces, Trans. Amer. Math. Soc.,, 100, 29-43 (1961) · Zbl 0102.32701
[5] Zarantonello, E. G., The closure of the numerical range contains the spectrum, Bull. Amer. Math. Soc.,, 70, 781-787 (1964) · Zbl 0137.32501
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