¹ R. J. Seeger, Proc. Nat. Acad. Sci. 17,301 (1931) writes for the idem symbol of all symbolic algebras of quantum theory I ≡ α {u, v} ≡ α (uv–vu) and a symmetry condition for the independent elements, q and p, namely uv²u – vu²v. Theories may disagree as to (1) identification of u and v (2) the unique interpretation of q and p, e.g., as operators, matrices, etc., and (3) the specification of α. Recent theories agree in setting u ≡ p, v – q, and .

² R. B. Lindsay, Proc. Nat. Acad. Sci. 17,420 (1931) has thus considered the equation of propagation for a small disturbance from equilibrium in a compressible fluid medium, which can be split up into two: (1) An equation of continuity ∇²φ = – ⋅ and (2) an equation of motion . [φ is the velocity potential, C is the velocity of propagation, and S is a quantity, called “condensation,” introduced in order to make possible the separation info two equations]. The second equation may be said to deal with the “particle” aspect, since one develops it for a “part of the medium considered as a whole.”