1. Arnold, V. I.: Sur une propriete topologique des applications globalement canoniques de la mecanique classique, C. R. Acad. Sci. Paris261, 3719?3722 (1965)
2. ?-: Mathematical methods of classical mechanics (Appendix 9), Nauka 1974; Engl. Transl. Berlin, Heidelberg, New York: Springer 1978
3. Aronszajn, N.: A unique continuation theorem for solutions of elliptic partial differential equations or inequalities of the second order. J. Math. Pures Appl.36, 235?249 (1957)
4. Atiyah, M. F., Singer, I. M.: The index of elliptic operators I, III. Ann. Math.87, 484?530 (1968);87, 546?604 (1968)
5. ??, ??: Index theory of skew adjoint Fredholm operators. Publ. Math. IHES37, 305?325 (1969)