[1] A. T. Dash: Joint essential spectra. Pacific J. Math., 64 (1976), 119-128. DOI 10.2140/pjm.1976.64.119 | MR 0425641 | Zbl 0329.47001

[2] R. G. Douglas: Banach algebra techniques in operator theory. Acad. Press, 1972. MR 0361893 | Zbl 0247.47001

[3] P. A. Fillmore, J. G. Stampfli, J. P. Williams: On the essential numerical range, the essential spectrum and a problem of Halmos. Acta Sci. Math. Szegal, 33 (1972), 179-192. MR 0322534 | Zbl 0246.47006

[4] P. R. Halmos: A Hilbert space problem book. Van Nostrand-Reinhold, 1967. MR 0208368 | Zbl 0144.38704

[5] T. Kato: Peturbation theory for linear operators. Springer Verlag, 1980.

[6] A. B. Patel: A joint spectral theorem for unbounded normal operators. J. Australian Math. Soc., (Series A) 34 (1983), 203-213. DOI 10.1017/S1446788700023235 | MR 0687326 | Zbl 0523.47010