1. S. Alexander, V. Kapovitch, A. Petrunin, Alexandrov geometry. http://www.math.psu.edu/petrunin/
2. D. Hilbert, The Foundations of Physics, (1915)
3. A. Lichnerowicz, Spineurs harmoniques. C. R. Acad. Sci. Paris, Série A 257, 7–9 (1963)
4. N. Hitchin, Harmonic Spinors. Adv. Math. 14, 1–55 (1974)
5. S. Stolz, Simply connected manifolds of positive scalar curvature. Ann. of Math. (2) 136, 511–540 (1992)