1. Lichnerowicz, A., Lett. Math. Phys. 13, 331?334 (1987).
2. Lichnerowicz, A., C.R. Acad. Sci Paris AB257, 7 (1963)
3. Baum, H., Spin Structures, Teubner, Leipzig, 1981, p. 138; Branson, T. and Kosmann-Schwarzbach, Y., Lett. Math. Phys. 7, 63 (1983).
4. Gromov, B. and Lawson, H. B., Ann. of Math. 111, 209?230 (1980).
5. Lusztig, G., J. Diff. Geom. 7, 229?256 (1971).