1. O. Bonnet, J. Ecole Polyth. 25(42), 1–151 (1867).
2. E. Cartan, Bull. Sci. Math. 66, 55–72; 74–85 (1942).
3. S. S. Chern, in Differential Geometry and Complex Analysis, Ed. by I. Chavel and H. M. Farkas (Springer-Verlag, Berlin, 1985), pp. 155–163.
4. X. X. Chen and Ch. K. Peng, in Differential Geometry and Topology (Tianjin, 1986–1987) (Springer-Verlag, Berlin, 1989), Vol. 1369, pp. 63–70.
5. G. Kamberov, Quadratic Differentials, Quaternionic Forms and Surfaces, arXiv:dg-ga/9712011 (1997).