Upper and lower Fredholm spectra. I
John J. Buoni, Robin E. Harte, Tony Wickstead · Proceedings of the American Mathematical Society · 1977
Joint upper and lower Fredholm spectra are defined for n -tuples of bounded linear operators, and the upper Fredholm spectrum is represented both as the simultaneous eigenvalues and as the simultaneous approximate eigenvalues of an n -tuple of operators obtained by a Berberian-Quigley construction.