1. This review is based in part on a review article which appeared in: Angew. Chem. Int. Ed. 12, 173 (1973)
2. For reviews see: Carbonium ions. Olah, G. A., Schleyer, P.v. R. (eds.), New York-London: Wiley-Interscience, Vol. 1, 1968, Vol. II, 1970, Vol. III, 1972, Vol. IV, 1973 and Vol. V, 1976.
3. Bethell, D., Gold, V.: Carbonium ions, an introduction, London-New York: Academic Press 1967
4. Also see: Olah, G. A.: Chem. Eng. News 45, 76 (March 27, 1967); Science, 168, 1298 (1970).
5. Existence is defined by Webster (Third New International Dictionary, Unabridged ed., Merriam Co., 1961, p. 796) as: The state or fact of having being, especially as considered independently of human consciousness