1. H. A. Kramers,Atti Congr. Int. Fis. Como 2 (1927) 545.
2. R. De L. Krönig,J. Opt. Soc. Am. Rev. Scient. Instrum. 12 (1926) 547.
3. Idem, Ned. Tijdschr. Natuurk 9 (1942) 402.
4. H. Bode, ?Network Analysis and Feedback Amplifier Design? (Van Nostrand, New York, 1945).
5. H. M. Nussenzveig, ?Causality and Dispersion Relations? (Academic Press, New York, 1972).