1. Neumanm, John von, The Mathematical Foundations of Quantum Mechanics, (trans, by R. Beyer), Princeton University Press, Princeton, 1955.
2. Feyerabend, P. K., ‘On the Quantum Theory of Measurement’, Observation and Interpretation, (ed. by S. Körner), Academic Press, New York, 1957, pp. 121–130.
3. Fano, U., ‘Description of States in Quantum Mechanics by Density Matrix and Operator Techniques’, Reviews of Modern Physics
29 (1957) 74ff.