1. F. Rellich, Math. Ann.(Leipzig) 116 (1939) 555;
2. F. Rellich, Math. Ann.(Leipzig) 117 (1940) 356.
3. F. Rellich, Perturbation Theory of Eigenvalue Problems, Gordon and Breach, New York, 1969.
4. T. Kato, J. Fac. Sci. Univ. Tokyo Ser. I 6 (1951) 145.
5. T. Kato, Perturbation Theory of Linear Operators, Springer, 2nd. Edition, 1976.