1. Choquet-Bruhat, Y.: Acta Math., 1952. See also a review article in: Gravitation: an introduction to current research, L. Witten (Ed.). New York: J. Wiley 1962.
2. Lichnerowicz, A.: Theories relativistes de la gravition et de l'electromagnétisme. Paris: Masson 1955.
3. Wheeler, J. A.: Article in: Relativity, groups and topology. New York: Gordon-Breach 1964.
4. Choquet-Bruhat, Y.: Article in: Batelle seattle recontres. New York: Benjamin 1968.
5. Leray, J.: Hyperbolic differential equations. Preprint, Princeton, 1952.