1. W. Rindler, Special Reletivity (Wiley-Interscience, Inc., New York, 1966), p.22.
2. H. Bondi, Relativity and Common Sense (Doubleday & Co., Inc., Garden City, N. Y., 1964).
3. R. Penrose in Relativity, Groups and Topology, C. and B. DeWitt, Eds. (Gordon and Breach Science Publ., Inc., New York, 1964), p. 565.
4. A. P. French, Special Relativity (W. W. Norton & Company, Inc., New York, 1968)
5. R. Resnick, Introduction to Special Relativity (John Wiley & Sons, Inc., New York, 1968)