1. M. A. Akivis, “The structure of two-component conjugate systems,” Tr. Geom. Semin.,1, 7–31 (1966).
2. M. A. Akivis, “On the theory of conjugate systems on multidimensional spaces,” in: Abstracts, International Mathematical Congress [in Russian], Section 9, Moscow (1966), pp. 20–21.
3. M. A. Akivis, “The structure of conjugate systems on multidimensional spaces,” Izv. Vyssch. Uchebn. Zaved., Mat., No. 10, 3–11 (1970).
4. M. A. Akivis and V. V. Ryzhkov, “Multidimensional surfaces of special projective types,” in: Proc. Fourth All-Union Mathematical Congress [in Russian], Nauka, Leningrad (1964), Vol. 2, pp. 159–164.
5. A. Besse, Einstein Manifolds, Vol. 1, Springer, Berlin (1987).