1. Apostol, C.: The correction by compact perturbation of the singular behavior of operators, Rev. Roumaine Math. Pures Appl. 21 (1976), 155?175.
2. Apostol, C.: The reduced minimum modulus, preprint, 1982.
3. Bart, H. and Lay, D. C.: The stability radius of a bundle of closed linear operators, Studia Math. 66 (1980), 307?320.
4. Förster, K.-H. and Kaashoek, M. A.: The asymptotic behaviour of the reduced minimum modulus of a Fredholm operator, Proc. Amer. Math. Soc. 49 (1975), 123?131.
5. Gohberg, I. C. and Krein, M. G.: The basic propositions on defect numbers, root numbers and indices of linear operators (Russian), Uspekhi Mat. Nauk 12, 2 (1957), 43?118. English translation: Amer. Math. Soc. Transl. (2) 13 (1960), 185?265.