1. J. von Neumann and E.P. Wigner, Z. Phys. 30 (1929), 467
2. G. Casati (editor) Chaotic behaviour in quantum systems, New York: Plenum 1985
3. T. Kato Perturbation Theory oj Linear Operators, Springer 1966
4. C.M. Bender and T.T. Wu, Phys. Rev. D7 (1973), 1620
5. P.E. Shanley, Ann. Phys. (N.Y.) 186 (1988), 292