1. K. O. Friedrichs, Bull. Am. Math. Soc.,61, 485–504 (1955).
2. J. Moser, Comm. Pure Appl. Math.,8, 251–278 (1955).
3. V. P. Maslov, Dokl. Akad. Nauk SSSR,111, No. 3, 531–534 (1956).
4. T. Kato, Perturbation Theory for Linear Operators, Springer-Verlag, New York, 1966.
5. C. C. Lin, The Theory of Hydrodynamical Stability, Cambridge University Press, 1955.