1. J.A. Stratton. “Electromagnetic Theory,” McGraw-Hill, New York, (1941), pp. 333–340.
2. H.M. Nussenzveig. “Causality and Dispersion Relations,” Academic, New York (1972), ch. 1.
3. G.R. Baldock and T. Bridgeman. “Mathematical Theory of Wave Motion,” Halsted, New York (1981), ch. 5.
4. E.T. Capson. “Asymptotic Expansions,” Cambridge University Press, London (1971), ch. 7-8.
5. F.W.J. Olver, Why steepest descents?, SIAM Rev. 12: 228 (1970).