1. J. VON NEUMANN: Mathematical Foundations of Quantum Mechanics, Princeton University Press, 1955.
2. G. LUDERS: Ann. Phys. (Leipzig) (6) 8, 322 (1951).
3. E.B. DAVIES and J.T. LEWIS: Commun. Math. Phys. 17, 239 (1970).
4. E.B. DAVIES: Quantum Theory of Open Systems, Academic Press, 1976.
5. M.D. SRINIVAS: Commun. Math. Phys. 71, 131 (1980).