1. V. I. Arnol'd, Supplementary Chapters of the Theory of Ordinary Differential Equations [in Russian], Nauka, Moscow (1978).
2. V. I. Arnol'd, Theory of Catastrophes [in Russian], Znanie, Ser. Mat., Kibern., No. 9, Moscow (1981).
3. V. I. Arnol'd, Ordinary Differential Equations [in Russian], Nauka, Moscow (1971).
4. V. I. Arnol'd, "Wavefront evolution and equivariant Morse lemma," Commun. Pure Appl. Math.,29, No. 6, 557?582 (1976).
5. L. Dara, "Singularites generiques des equations differentielles multiformes," Bol. Soc. Bras. Math.,6, No. 2, 95?129 (1975).