1. Wheeler, J. A., in: Gravitation and Relativity (edit. by Chiu, H. -Y. and Hoffmann, W. F.), (Benjamin, New York, 1966).
2. Thirring, W., Ann. Phys., 16, 96 (1961).
3. Mittelstaedt, P., and Barbour, J. B., Z. Phys., 203, 82 (1967).
4. Einstein, A., The Meaning of Relativity, chapter 3 (Methuen, London, 1922).
5. Bondi, H., Cosmology (Cambridge University Press, 1960).