1. C. Apostol, C. Foias, and D. Voiculescu, "Some results on nonquasitriangular operators", II,Rev. Roum. Math. Pures et Appl., 18 (1973), 159–181.
2. C. Apostol, C. Foias, and D. Voiculescu, "Some results on nonquasitriangular operators", III,Rev. Roum. Math. Pures et Appl., 18, (1973), 309–324.
3. C. Apostol, C. Foias, and D. Voiculescu, "Some results on nonquasitriangular operators", IV,Rev. Roum. Math. Pures et Appl., 18 (1973), 487–514.
4. C. Apostol and B. Morrel, "On uniform approximation of operators by simple models, "Indiana University Math. J., 26 (1977), 427–442.
5. N. J. Bezak and M. Eisen, "Continuity properties of operator spectra",Canad. J. Math., 29 (1977), 429–437.