1. Atkinson, F.V.: The normal solubility of linear operators in normed spaces, Mat. Sbornik, N.S. 28 (70), pp. 3?14 (Russian)
2. Dieudonné, J.: Sur les homomorphismes d'espaces normés, Bull. Sci. Math. (2), 67 (1943), pp. 72?84.
3. Gohberg, I.C.: On linear equations in normed spaces, Dokl. Akad. Nauk, SSR (N.S.) 76 (1951), pp. 477?480 (Russian)
4. Gohberg, I.C. and Krein, M.: The basic propositions on defect numbers, root numbers and indices of linear operators, A.M.S. Transl. (2), 13 (1960), pp. 185?264 (Uspehi Mat. Nauk (N. S.), 12 (1957), pp. 43?118).
5. Kato, T. Perturbation theory for nullity, deficiency and other quantities of linear operators, Journ. Anal. Math., 6, (1958), pp. 261?322.