1. A. Einstein: Verh. Dt. Phys. Ges. 19, 82 (1917)
2. M.C. Gutzwiller: Chaos in Classical and Quantum Mechanics (Springer, New York, 1990)
3. M.C. Gutzwiller: J. Math. Phys. 12, 343 (1971)
4. P. Cvitanović, R. Artuso, R. Mainieri, G. Tanner and G. Vattay: Chaos: Classical and Quantum, ChaosBook.org (Niels Bohr Institute, Copenhagen 2005)
5. M. Brack, R.K. Bhaduri: Semiclassical Physics (Addison-Wesley, Reading, MA, 1997)