1. G. Birkhoff andJ. von Neumann:Annals of Mathematics,37, 823 (1936) ; for a brief discussion of Boolean algebra seeG. Birkhoff andS. MacLane:A Survey of Modern Algebra (New York, 1944), Chap. XI.
2. See, for example, ref. (1), pp. 320–323.
3. J. von Neumann, ref. (8), pp. 187 ff. These classification measurements correspond to themeasurements of the first kind discussed byW. Pauli:Handbuch der Physik,24, 152 ff. (1933).
4. E. C. Kemble:Fundamental Principles of Quantum Mechanics w York, 1937), p. 327.
5. G. Temple:The General Principles of Quantum Theory (London, 1951), p. 25.