1. S. P. Novikov, Soviet Math. Dokl. 24, 222-226, (1981)
2. S. P. Novikov, Russ. Math. Surveys 37 (5), 1-56, (1982)
3. S. P. Novikov, Soviet Math. Dokl. 33 (5), 551-555, (1986)
4. A. V. Pazhitnov, Soviet Math. Dokl. 35, 1-2, (1987)
5. E. Witten, Supersymmetry and Morse theory, J. Differential Geom. 17, 661-692, (1982)