Schmidt, Bernd Spektrum, numerischer Wertbereich und ihre Maximumprinzipien in Banachalgebren. (German) Zbl 0186.19701 Manuscr. Math. 2, 191-202 (1970). PDFBibTeX XMLCite \textit{B. Schmidt}, Manuscr. Math. 2, 191--202 (1970; Zbl 0186.19701) Full Text: DOI EuDML
Berberian, S. K. Some conditions on an operator implying normality. (English) Zbl 0176.11301 Math. Ann. 184, 188-192 (1970). PDFBibTeX XMLCite \textit{S. K. Berberian}, Math. Ann. 184, 188--192 (1970; Zbl 0176.11301) Full Text: DOI EuDML