1. See, for example,E. C. Titchmaksh:Theory of Fourier Integrals (Oxford, 1937), p. 127–128 and p. 125; orJ. S. Toll:Phys. Rev.,104, 1760 (1956); orJ. Hilgevoobd:Dispersion Belations and Causal Description (Amsterdam, 1960), p. 34–36.
2. For a simple proof, seeN. Page:Physical Mathematics (New York, 1958), p. 224.
3. J. S. Toll:Phys. Rev.,104, 1760 (1956), Section 4, and the mathematical literature quoted therein.
4. M. Born andE. Wolf:Principles of Optics (London and New York, 1956), chapt. 10.
5. E. Wolf:Proc. Phys. Soc,80, 1269 (1962).