1. ATIYAH, M.F., HITCHIN, N.J. & SINGER, I.M. Self-duality in four dimensional Riemannian geometry, Proc. R. Soc. Lond. A362, (1978), 425–461
2. CARTAN, E. Sur une classe d’espaces de Weyl, Ann. Ec. Norm 60 (1943), 1–16
3. DOUGLAS, J. The general geometry of paths, Ann. Math. 29 (1927), 143–168
4. EDDINGTON, A.S. The mathematical theory of relativity, Cambridge Univ. Press, Cambridge (1922)
5. EISENHART, L.P. A treatise on the differential geometry of curves and surfaces, Ginn & Co., Boston (1909)