1. V. I. Arnold, A. N. Varchenko, and S. M. Gusein-Zade, Singularities of Differentiable Maps. Vol. 1: Classification of Critical points, Caustics and Wave Fronts, Birkhäuser, Boston, Mass., 1985.
2. V. I. Arnold, A. N. Varchenko, S. M. Gusein-Zade, Singularities of Differentiable Maps. Vol. 2: Monodromy and Asymptotics of Integrals, Birkhäuser, Boston, Mass., 1988.
3. M. V. Fedoryuk, Mountain Pass Method, Nauka, Moscow, 1983 (Russian).
4. M. V. Fedoryuk, Asymptotics. Integrals and Series, Nauka, Moscow, 1987 (Russian).
5. V. I. Arnold, Catastrophe Theory, Springer-Verlag, Berlin, 1992.