1. Dunford, N., and Schwartz, J. T., Linear Operators, Vol. 1: General Theory, Interscience, New York (1958).
2. Wermer, J., The existence of invariant subspaces, Duke Math. J., 19 (4): 615 - 622 (1952).
3. Lyubich, Yu. I., and Matsaev, V. I., On operators with a divisible spectrum, Matem. Sborn., 56(98) (4): 533 - 468 (1962).
4. Levinson, N., Gap and Density Theorems (AMS Colloquium Publications, No. 26), American Mathematical Society, Providence, R. L (1940).
5. Gurarii, V. P. On Levinsons theorem concerning normal families of analytic functions, this volume, p. 124,