1. V. I. Arnol'd, "Characteristic class which occurs in the quantization condition," Funkts. Anal. Prilozhen.,1, No. 1, 1–14 (1967).
2. V. I. Arnol'd, "Singularities of system of rays," Usp. Mat. Nauk,38, No. 2, 77–147 (1983).
3. V. I. Arnol'd, A. N. Varchenko, and S.-M. Gusein-Zade, Singularities of Differentiable Maps [in Russian], Vol. 1, Nauka, Moscow (1984).
4. O. P. Shcherbak, "Projective duality of spaces curves and Legendre singularities," Trudy Tbilis. Univ.,232–233, Nos. 13–14, 280–336 (1982).
5. M. Gromov, "Pseudo holomorphic curves in symplectic manifolds," Invent. Math.,82, No. 5, 307–347 (1985).