1. Apostol, C., Fialkow, L., Herrero, D.A., Voiculescu, D.: Approximation of Hilbert Space Operators, Vol II, Research Notes in Mathematics, 102, Pitman Books Limited, UK (1984)
2. Apostol C., Foiaş C., Pearcy C.: That quasinilpotent operators are norm-limits of nilpotent operators revisited. Proc. Am. Math. Soc. 73(1), 61–64 (1979)
3. Apostol C., Foiaş C., Voiculescu D.: On the norm-closure of nilpotents, II. Rev. Roum. Math. Pures et Appl. 19, 549–577 (1974)
4. Apostol C., Salinas N.: Nilpotent approximations and quasinilpotent operators. Pac. J. Math. 61(2), 327–337 (1975)
5. Apostol C., Voiculescu D.: On a problem of Halmos. Rev. Roum. Math. Pures Appl. 19, 283–284 (1974)