1. L. Landau and E. Lifšic, Quantum Mechanics, 2nd ed., part I, Chap. 7, Pergamon Press, Oxford 1965.
2. H. Mehlberg, The Reach of Science, Chap. 17, Toronto 1958.
3. In short, we shall sometimes refer to ‘correct operations’ taking it implicite for granted that they are correct in relation to a given method.
4. Op. cit., p. 13.
5. The word ‘continuity’ is maybe not the best because of the discrete, as a rule, character of the set K; but I do not know of any better term.